Make `blade_rep` and `base_rep` non mutating

[eric-wieser]

Previously this would cause weird and surprising swaps between basis representations.

[eric-wieser]

Now that these don't mutate state, there is no need to try and undo the swap between representations

[eric-wieser]

There are other places in this file that simply did not swap between representations, which is what motivated the change

[codecov]

Codecov Report

Merging #60 into master will increase coverage by 0.07%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   66.54%   66.61%   +0.07%     
==========================================
  Files           8        8              
  Lines        4899     4895       -4     
==========================================
+ Hits         3260     3261       +1     
+ Misses       1639     1634       -5

Impacted Files	Coverage Δ
galgebra/mv.py	61.79% <76.92%> (+0.27%)	⬆️
galgebra/utils.py	55.55% <0%> (-11.12%)	⬇️
galgebra/ga.py	74.39% <0%> (+0.06%)	⬆️

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[utensil]

Thank you for fixing this and the coverage increased a bit too. 👍