more custom dp scores for faster contractions

[jcmgray]

Description

This adds more custom functions to the dp solver as outlined in #171. For example:

opt = oe.DynamicProgramming(minimize='combo')

should give better real world performance in many cases than the pure minimized flop cost alone.
Also included is a couple of 'micro' optimizations for the path finders performance - namely, making some inner loop calls functions not methods.

Todos

• at some point might be nice to add these to the random greedy path as well

Questions

• I tried the black formatter and it showed lots of changes across many files - currently have just committed those on the files modified...? Am I invoking it wrong? (edit: might just be that the conda black is a bit out-of-date)

Status

• Ready to go

[codecov]

Codecov Report

Merging #181 (6846f14) into master (6a30cf5) will decrease coverage by 0.07%.
The diff coverage is 95.16%.

[dgasmith]

Looks great! Can we add a few notes on the DP optimizer's new minimizers in the docs?

[dgasmith]

LGTM once mypy is worked through. Let me know if you would like a hand!