Avoid checking whether the basis blades are blades

[eric-wieser]

Cherry-picked from @meuns' branch (#68)
We can skip a check here, since we know Ga.mv_blades will contain only blades.

No behavior change, but likely a small performance boost.

[codecov]

Codecov Report

Merging #275 into master will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
- Coverage   72.74%   72.71%   -0.03%     
==========================================
  Files          11       11              
  Lines        4472     4472              
==========================================
- Hits         3253     3252       -1     
- Misses       1219     1220       +1     

Impacted Files	Coverage Δ
galgebra/mv.py	74.30% <100.00%> (ø)
galgebra/ga.py	76.87% <0.00%> (-0.10%)	⬇️

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