This commit collects the changes in `GeneratorObjects-01-04-00-19`
and `GeneratorObjectsTPCnv-00-06-10-05`. Plus a number of additional
changes:
 - added a method to directly set the `ExtendedBarCode` member
variable of `HepMcParticleLink`, as this was needed by the
`TRTUncompressedHitCollection` TP Converters.
 - added some missing braces to make contents of for loop clearer
 - added the possibility to write old `HepMcParticleLink` format
using event position instead of event number (cherry picked from
commit 31207b97f0453496aaa8794883505c562dee36d4)
 - added a unit test for `HepMcParticleLinkCnv_p2` and extended the
unit test for `HepMcParticleLinkCnv_p1`
 - Changing the `ExtendedBarCode` should force the `HepMcParticleLink`
to re-find the `HepMC::GenParticle`. Previously calling
`HepMcParticleLink::setExtendedBarcode` did not force the `HepMcParticleLink`
instance to refind the `HepMC::GenParticle` to which it should be linking.
This could cause the pointer to be inconsistent with the information cached
in the `HepMcParticleLink`.
 - Ensure that all member variables are properly initialized in the
`HepMcParticleLink` default constructor. Previously `m_particle`
and `m_have_particle` were not initialized properly.

Former-commit-id: 8607d473dddbd5d9921b41f64be2ed52c72a4cdb

The `Athena_test::initMcEventCollection` helper function calls
`Athena_test::initGaudi` then creates an `McEventCollection` in
StoreGate then fills it will some dummy information. It also
returns a vector of `HepMC::GenParticle` pointers which can then
be used to create valid `HepMcParticleLink` objects.


Former-commit-id: c8cb2186fe52b69c11c6969637b58c9b5d493673

Add initial unit tests for HepMcParticleLinkCnv_p1, McEventCollectionCnv_p4 and McEventCollectionCnv_p5

Initial unit tests for `HepMcParticleLinkCnv_p1`, `McEventCollectionCnv_p4`
and `McEventCollectionCnv_p5`. These should work before the `HepMcParticleLink`
migration too, so adding them as a separate commit.


Former-commit-id: 84220a3ea6fa5f25632a0552a8bbbacb4cd564fb

This commit adds the new `HepMcParticleLink_p2` persistent format
that will in future be written out by jobs run `21.3` and `master`.
These changes are for forward compatibility only, so while a P->T
method is defined, the T->P method of `HepMcParticleLinkCnv_p2`
just throws an exception saying that writing `HepMcParticleLink_p2`
is not supported in this release. See ATLASSIM-3457.


Former-commit-id: 0cd6453e36785d5128d7d7b3bc50df525a372abb

Former-commit-id: 569384a325a5a40ab219eeef4c5167d591386542

'Set the store pointer in the link, if needed.  Remove #define private.' (GeneratorObjectsTPCnv-00-06-11)

	* Tagging GeneratorObjectsTPCnv-00-06-11
	* src/HepMcParticleLinkCnv_p1.cxx: Set the store pointer in the
	link, if needed.
	* GeneratorObjectsTPCnv/McEventCollectionCnv_p1.h: Remove #define
	private.


Former-commit-id: beaacc7c328170499a633681907c7964b32a219c

      * src/McEventCollectionCnv_p4.cxx, src/McEventCollectionCnv_p5.cxx : moved HepMCWeightSvc to new package
      * Tagging as  GeneratorObjectsTPCnv-00-06-10

2015-07-14  John Chapman  <John.Chapman@cern.ch>

	* src/McEventCollectionCnv_p4.cxx,
	GeneratorObjectsTPCnv/McEventCollectionCnv_p4.h - Using
	HepMCWeightSvc to store weight names in metadata, as this version
	is still used in a few places. AGENE-984
	* Tagging as GeneratorObjectsTPCnv-00-06-09

2015-06-25 Will Buttinger <will@cern.ch>

	* src/McEventCollectionCnv_p5.cxx,
	GeneratorObjectsTPCnv/McEventCollectionCnv_p5.h - Using
	HepMCWeightSvc to store weight names in metadata. AGENE-984
	* Tagging as GeneratorObjectsTPCnv-00-06-08

2015-06-17  John Chapman  <John.Chapman@cern.ch>

...
(Long ChangeLog diff - truncated)

	* Tagging GeneratorObjectsTPCnv-00-06-06.
	* cmt/requirements: Fix checkreq warnings.

2015-01-31  scott snyder  <snyder@bnl.gov>

	* Tagging GeneratorObjectsTPCnv-00-06-05.
	* Remove unused ARA converter dictionaries.

	* adding p5 files

src/McEventCollectionCnv_p5.cxx - clear Flow member variable of new transient GenParticle before filling it from the persistent information. Test fix for ATLASSIM-528. Tagged GeneratorObjectsTPCnv-00-06-03-01 (GeneratorObjectsTPCnv-00-06-03-01)