Add Computer.eval()

[khaeru]

• Add a method Computer.eval() that allows to define tasks and keys using a subset of Python syntax.
• Add .computations.sub.
• Test and close some small bugs (see sidebar).

PR checklist

• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[codecov-commenter]

Codecov Report

Merging #97 (87e8c49) into main (0f577d2) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              main       #97    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           42        43     +1     
  Lines         3383      3524   +141     
==========================================
+ Hits          3383      3524   +141     

Files Changed	Coverage Δ
genno/compat/xarray.py	100.00% <100.00%> (ø)
genno/computations.py	100.00% <100.00%> (ø)
genno/core/attrseries.py	100.00% <100.00%> (ø)
genno/core/computer.py	100.00% <100.00%> (ø)
genno/core/eval.py	100.00% <100.00%> (ø)
genno/core/key.py	100.00% <100.00%> (ø)
genno/testing/__init__.py	100.00% <100.00%> (ø)
genno/tests/core/test_attrseries.py	100.00% <100.00%> (ø)
genno/tests/core/test_computer.py	100.00% <100.00%> (ø)
genno/tests/core/test_quantity.py	100.00% <100.00%> (ø)
... and 1 more