Create a symlink to the built Python binary modules so that they can be
imported as `Package.Module` (same as it is the case once they are
installed).

plus minor fixes

Before this change, RootHistogramSink always puts a histogram
in a top level directory named according to the component name.
This change allows a histogram to be saved in a custom directory
by naming it with a leading slash. In that case, the component
name is ignored.

Plus example of histogram customization (and extension of the unit tests) on a log log histogram

One used to be able to use e.g. a 2D histogram with a syntac like `++histo[singleNumber]`
The second and missing number wa simply set to 0.

This is now creating a compilation error and a test was added to check that error

so that things work in a super project build

Remove the deprecated StatusCode "checking" code. This code is
non-functional and marked with a deprecation warning since %v36r0
(see !989). Also delete deprecated implicit type conversion
constructors.

- and add a `#define` to simplify migration

The `TObject::InheritsFrom` call can be quite slow. Since we already
deduce the object type (`TH1`, `TTree`, etc.) during registration, save
this information and use it instead of re-deriving it every time.
Also avoid calling `updateFiles` if no TTrees are registered.

Both these changes significantly speed up the `EndEvent` incident
handling for jobs with large (>10k) number of histograms.

Also re-organize `struct THistID` to avoid padding and delete unused
`reset` method.