AttributeError: Failed to get attribute TPyMultiGenFunction from ROOT for ROOT 6.22

[guiguem]

Describe the bug
When using ROOT 6.22, one gets the following error when loading morpho:

RooFit v3.60 -- Developed by Wouter Verkerke and David Kirkby
                Copyright (C) 2000-2013 NIKHEF, University of California & Stanford University
                All rights reserved, please read http://roofit.sourceforge.net/license.txt

Traceback (most recent call last):
  File "fake_data_stan_analysis.py", line 20, in <module>
    from morpho.utilities import morphologging
  File "/usr/local/lib/python3.8/site-packages/morpho/__init__.py", line 24, in <module>
    module = loader.find_module(name).load_module(name)
  File "/usr/local/lib/python3.8/site-packages/morpho/processors/__init__.py", line 7, in <module>
    from . import diagnostics
  File "/usr/local/Cellar/root/6.22.02/lib/root/ROOT/_facade.py", line 121, in _importhook
    return _orig_ihook(name, *args, **kwds)
  File "/usr/local/lib/python3.8/site-packages/morpho/processors/diagnostics/__init__.py", line 6, in <module>
    from .CalibrationProcessor import CalibrationProcessor
  File "/usr/local/Cellar/root/6.22.02/lib/root/ROOT/_facade.py", line 121, in _importhook
    return _orig_ihook(name, *args, **kwds)
  File "/usr/local/lib/python3.8/site-packages/morpho/processors/diagnostics/CalibrationProcessor.py", line 16, in <module>
    from morpho.processors import BaseProcessor
  File "/usr/local/Cellar/root/6.22.02/lib/root/ROOT/_facade.py", line 121, in _importhook
    return _orig_ihook(name, *args, **kwds)
  File "/usr/local/lib/python3.8/site-packages/morpho/processors/__init__.py", line 11, in <module>
    from . import sampling
  File "/usr/local/Cellar/root/6.22.02/lib/root/ROOT/_facade.py", line 121, in _importhook
    return _orig_ihook(name, *args, **kwds)
  File "/usr/local/lib/python3.8/site-packages/morpho/processors/sampling/__init__.py", line 11, in <module>
    from .PyBindRooFitProcessor import PyBindRooFitProcessor
  File "/usr/local/Cellar/root/6.22.02/lib/root/ROOT/_facade.py", line 121, in _importhook
    return _orig_ihook(name, *args, **kwds)
  File "/usr/local/lib/python3.8/site-packages/morpho/processors/sampling/PyBindRooFitProcessor.py", line 22, in <module>
    class PyFunctionObject(ROOT.TPyMultiGenFunction):
  File "/usr/local/Cellar/root/6.22.02/lib/root/ROOT/_facade.py", line 163, in _fallback_getattr
    raise AttributeError("Failed to get attribute {} from ROOT".format(name))
AttributeError: Failed to get attribute TPyMultiGenFunction from ROOT

Morpho version
v2.7.1

Additional context
This class was removed from ROOT and replaced: see https://root.cern/doc/v622/release-notes.html#release-6.2202

[nsoblath]

https://root.cern/doc/v622/release-notes.html#pyroot

It looks like the inheritance of PyFunctionObject needs to be changed to be from ROOT.IMultiGenFunction instead of ROOT.TPyMultiGenFunction. Probably there should be some detection of whether the latter exists to do the inheritance right.

[guiguem]

Indeed, this is what I meant in the additional context section.
I think this is easy to change (they also give an example on how to adapt people's code), this needs to be tested though :)