Introduce UidModel subclass of HasProperties

[fwkoch]

UidModel primarily exists to improve serialization. By adding a uid to all objects, they can be serialized into a flat dictionary that only contains each object once, with all references done by uid (as opposed to the nested structure of HasProperties serialization). This also allows recursive self-references to be serialized. This is based on a class written for the OMF library: https://github.com/GMSGDataExchange/omf/blob/v0.9.0/omf/base.py#L12 - see #128

HasProperties serialization:

UidModel serialization:

[codecov-io]

Codecov Report

Merging #149 into master will decrease coverage by 1.17%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
- Coverage   96.64%   95.46%   -1.18%     
==========================================
  Files          14       14              
  Lines        1756     1788      +32     
==========================================
+ Hits         1697     1707      +10     
- Misses         59       81      +22

Impacted Files	Coverage Δ
properties/__init__.py	80% <ø> (ø)	⬆️
properties/base/base.py	90.11% <50%> (-8.53%)	⬇️

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[fwkoch]

A similar but updated implementation is found in #234