Remove yet more special-casing of the hestenes dot product

[eric-wieser]

Continues from #134 and 6cc530c

This also changes some sympy zeros to multivector zeros, which means that (A | scalar) ^ (B | scalar) is not a TypeError, as it was before:

>>> s = g.mv(1, 'scalar')
>>> (ex | s) ^ (ey | s)
TypeError: unsupported operand type(s) for ^: 'Zero' and 'Zero'

[codecov]

Codecov Report

Merging #154 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
- Coverage   67.14%   67.12%   -0.03%     
==========================================
  Files           8        8              
  Lines        4761     4758       -3     
==========================================
- Hits         3197     3194       -3     
  Misses       1564     1564

Impacted Files	Coverage Δ
galgebra/mv.py	62.8% <100%> (-0.07%)	⬇️

---

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 a947dd1...09147b1. Read the comment docs.

[utensil]

LGTM.