Improve performance of arithmetic on MultiVector pairs

[eric-wieser]

The previous code was optimized for MultiVector <op> something_else, incurring an expensive isinstance(other, numbers.Number) check.

This changes it to check isinstance(other, MultiVector) first, which is the case that is going to be most frequent.

Extracted from #280, since it's a tiny patch

[codecov]

Codecov Report

Merging #283 into master will increase coverage by 0.08%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
+ Coverage   63.06%   63.14%   +0.08%     
==========================================
  Files          50       50              
  Lines        7294     7294              
  Branches     1033     1033              
==========================================
+ Hits         4600     4606       +6     
+ Misses       2545     2540       -5     
+ Partials      149      148       -1     

Impacted Files	Coverage Δ
clifford/_multivector.py	81.23% <60.00%> (ø)
clifford/test/test_g3c_tools.py	79.67% <0.00%> (+0.47%)	⬆️
clifford/tools/g3c/model_matching.py	42.77% <0.00%> (+1.20%)	⬆️

---

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 a5c9431...d8a8309. Read the comment docs.