…PDUs

Instead of duplicating code already in the above two classes,
existing classes that implement the same functionality use them instead.

Access to the underlying attributes is provided through @property-decorated methods.

Affected classes:
Data Query PDU
Action Request PDU
Data PDU
Set Data PDU
Event Report PDU
Comment PDU
... and their Reliable counterparts

Remove unnecessary whitespace, and add any where required to follow PEP8 recommendations.

…ically

Instead of storing the number  of fixed datums and variable datums as an attribute,
calculate it from the list of fixed/variable datums when required.

The original attribute names are still accessible, through class properties.

This style is no longer required in Python 3; all classes that do not inherit another superclass
are automatically assumed to subclass object.

These are moved from the __init__() to class attributes.
This pattern makes it less likely for users to accidentally initialise the class with the
wrong recordType or recordLength, where these values are known beforehand.

Class attributes are accessible by instances of the class, so the serialize() and parse()
methods are unaffected.

### Further changes

- added type annotations to attributes, marking them as being an enum, int, or float,
  along with their size in bits
- fixed init types for some attributes
- removed some unnecessary comments
- added table UID in comments for some enums

These are calculated from the list of fixed/variable datum records instead.
The attributes are still accessible as properties.

…xedDatum or VariableDatum

Changes:
- removed redundant comments, whitespace
- added type aliases for Python int, float, bytes
- added type annotation for init parameters, padding
- added UID number in comments for enums from SISO-REF-010
- changed superclass attribute values (pduType, protocolFamily) from init to class attributes
- Fixed some parsing of arrays

The type aliases are moved to their own file, types.py, to avoid a dependency loop.

This is considered safer as it automatically releases the file handle
if the program should crash before f.close() is reached.