Simplify the common printing methods between classes

[eric-wieser]

It's easier to make these a base class than it is to import them separately at each layer.

This also:

• Fixes Sdop._repr_latex_ and Pdop._repr_latex_, which forgot to enter math mode (by removing their broken implementations).
• Removes Dop.title, which was always None, never set, and not used until a very recent commit.
• Simplifies _FmtResult to always have a title.

[codecov[bot]]

Codecov Report

Merging #386 into master will decrease coverage by 0.10%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #386      +/-   ##
==========================================
- Coverage   77.68%   77.58%   -0.11%     
==========================================
  Files          16       16              
  Lines        4248     4220      -28     
==========================================
- Hits         3300     3274      -26     
+ Misses        948      946       -2     

Impacted Files	Coverage Δ
galgebra/dop.py	91.07% <100.00%> (+1.32%)	:arrow_up:
galgebra/lt.py	59.30% <100.00%> (-0.47%)	:arrow_down:
galgebra/mv.py	74.91% <100.00%> (-0.30%)	:arrow_down:
galgebra/printer.py	78.75% <100.00%> (-0.27%)	:arrow_down:

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