Pickling, hash & equality details

[MichaelClerx]

See #842

• Pickling an expression now raises an error which suggests an alternative way to serialise expressions.
• myokit.parse_expression() now accepts a model as a context.
• model comparison with == is now back to the default is implementation. The old == overload is available as Model.is_similar
• Model.get() and Component.get() raise errors if objects from a different model are passed in
• Hashes for equation and quantity were fixed, equations are now immutable.

[codecov]

Codecov Report

Merging #849 (e469f09) into main (7742f84) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #849   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          104       104           
  Lines        20335     20345   +10     
=========================================
+ Hits         20335     20345   +10     

Impacted Files	Coverage Δ
myokit/_protocol.py	100.00% <ø> (ø)
myokit/_expressions.py	100.00% <100.00%> (ø)
myokit/_model_api.py	100.00% <100.00%> (ø)
myokit/_parsing.py	100.00% <100.00%> (ø)
myokit/_unit.py	100.00% <100.00%> (ø)

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