with obvious adaptations

set correctly the path to the compiler

See merge request !310

- fixed override of LCG version in toolchain.cmake
- added runtime dependency on libpng (for ROOT)
- renamed property Atomic of GaudiSequencer to Sequential (to use !305)
- updated test reference
- fix to CTestXML2HTML
- updated Gitlab-CI configuration
- (temporary) workaround for GAUDI-1295

See merge request !306

required to be able to use !305

Prepare v28r2

See merge request !307

Sequential scheduling of an algorithm set (native support)

See merge request !305

needed with ROOT and LCG 88

Hide new proprty "Atomic" in tests

See merge request !301

fixed compilation with Clang

See merge request !294

See merge request !299

See merge request !304

allow the new scheduler-based mechanism of algorithms sequencing be controlled by AthSequencer property 'Sequential'

migrate from the concept of 'laziness', inherited from the era of GaudiSequencers', to a more meanignful concept of a 'prompt decision' (the change affected the PrecedenceRulesGraph only)

rename the root decision hub of the graph of precedence rules from 'EVENT LOOP' to 'RootDecisionHub'.

 o add explicit typing of a decision hub node as parallel/sequential (before it was considered as parallel by default, could be made 'sequential' with few dirty tricks);
 o make precedence graph traversals respect the parallel/sequential flag;
 o I also took this occasion to re-factor the bottom-up part of CF graph traversal making it based on graph visitor pattern instead of recursive one.

changed default LCG version to 88

See merge request !303

Closes GAUDI-1293

See merge request !296

See merge request !300

the property was added with !287