Please implement the conversion of full OpenMath objects with the OMOBJ tag

[MarcoCostantini]

The OpenMath standard requires the OMOBJ tag, and hence, when converting from OpenMath to Python, py-openmath should accept also OpenMath objects with that tag, even if that tag is not generated when converting in the opposite direction.

Implementing the OMOBJ tag is easy, just take the child of the OMOBJ node, and call again converter.to_python applied to it.

>>> converter.to_python( decoder.decode_bytes( b'<OMOBJ xmlns="http://www.openmath.org/OpenMath" version="2.0" cdbase="http://www.openmath.org/cd"><OMI>3</OMI></OMOBJ>', snippet=True ) )
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.8/dist-packages/openmath/convert.py", line 149, in to_python
    raise ValueError('Cannot convert object of class %s to Python.' % omobj.__class__.__name__)
ValueError: Cannot convert object of class OMObject to Python.
>>>