v0.4.0 updates, major changes to component APIs

[mcgibbon]

Some of the main ideas of things that are changing, particularly breaking changes:

• Components are written by writing an array_call method for the subclass which takes in a numpy array state satisfying the component's input properties, and returns numpy arrays satisfying the output properties. The base class automatically handles unwrapping and wrapping arrays in its __call__ implementation.
• Similarly, TimeSteppers are written by writing a _call method, which has the same signature as the previous __call__ method.
• ScalingWrapper, UpdateFrequencyWrapper, and TendencyInDiagnosticsWrapper have been removed, and their functionality is implemented in base component classes.
• 'time' must now always be present in the model state dictionary.
• TimeSteppers should be called using individual Prognostics as args, rather than using a list.
• TimeSteppers and Composites now properly have properties dictionaries implemented

Update: These bullets are no longer entirely accurate. Check HISTORY.rst in the latest version for complete changes.

[mcgibbon]

I'm still working on some of these changes, but they've mostly been implemented already. I still have to write tests to confirm that TimeStepper, DiagnosticComposite, and PrognosticComposite have accurate properties dictionaries, and I still have to update documentation. I should also remember to describe get_constants_string somewhere in the documentation.

[codecov-io]

Codecov Report

Merging #26 into master will increase coverage by 12.09%. The diff coverage is 90.63%.

@@            Coverage Diff             @@
##           master     #26       +/-   ##
==========================================
+ Coverage      76%   88.1%   +12.09%     
==========================================
  Files          15      23        +8     
  Lines        1313    2026      +713     
==========================================
+ Hits          998    1785      +787     
+ Misses        315     241       -74

Impacted Files	Coverage Δ
sympl/_core/get_np_arrays.py	100% <100%> (ø)
sympl/_core/dataarray.py	94.73% <100%> (ø)
sympl/_core/units.py	97.22% <100%> (+1.06%)	:arrow_up:
sympl/_core/state.py	100% <100%> (ø)
sympl/_core/exceptions.py	100% <100%> (ø)	:arrow_up:
sympl/_components/plot.py	24.24% <100%> (ø)	:arrow_up:
sympl/_components/__init__.py	100% <100%> (ø)	:arrow_up:
sympl/__init__.py	100% <100%> (ø)	:arrow_up:
sympl/_core/constants.py	83.69% <33.33%> (-6.43%)	:arrow_down:
sympl/_core/time.py	81.48% <80%> (-2.52%)	:arrow_down:
... and 25 more

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