Improve typing of arithmetic, Quantity.shift(), Quantity.to_dataframe()

[khaeru]

This adds explicit arithmetic so that:

x = 4.2 * Quantity(…)

…works and satisfies mypy.

Also:

• Improve typing of Quantity.shift().

PR checklist

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

[codecov-commenter]

Codecov Report

Merging #55 (f133e00) into main (d4f0d4b) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #55   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           36        36           
  Lines         2605      2628   +23     
=========================================
+ Hits          2605      2628   +23     

Impacted Files	Coverage Δ
genno/computations.py	100.00% <100.00%> (ø)
genno/core/attrseries.py	100.00% <100.00%> (ø)
genno/core/quantity.py	100.00% <100.00%> (ø)
genno/core/sparsedataarray.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)
genno/tests/test_computations.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d4f0d4b...f133e00. Read the comment docs.