Update property documentation

[mwatts15]

New-style properties (declared in class body) can have documentation attached to them whereas old-style properties (declared in __init__) did not. This allows for automation of documentation generation for those fields by defining docstrings and setting the __doc__ attribute as appropriate. A few classes have already been updated. In addition there are attributes of the property (like its value_type and inverse_of) which are now accessible for documentation generation after module load: these should be added to the doc string.

Example of a new-style properties:

class Clock(DataObject):
    hour = DatatypeProperty() # <--- new-style property
    ''' The hour of the clock ''' # <-- documentation for the property

    minute = DatatypeProperty()
    ''' The minute of the :py:attr:`hour` of the clock '''

    maker = ObjectProperty(value_type=ClockManufacturer) # undocumented

It may not be clear what documentation is appropriate to each attribute, but the first task is just to identify which attributes are lacking documentation.

[mwatts15]

The latest documentation is hosted here: https://pyopenworm.readthedocs.io/en/latest/