Split general contractions into blocks whenever possible

[susilehtola]

This pull request makes the BSE split general contractions into blocks whenever possible.

For example, if the contraction pattern looks like

x x x x 0 0
x x x x 0 0
x x x x 0 0 
x x x x 0 0
0 0 0 0 y y
0 0 0 0 y y

this will be identified by the code, and the general contraction will be split into two shells, consisting of the x and y entries, respectively.

A special case of the procedure concerns completely free primitives

x x x x 0 0
x x x x 0 0
x x x x 0 0 
0 0 0 0 y 0
0 0 0 0 0 z

which will from now on not be included in the generally contracted shell; instead, they will be contained in their own shells.

The PR doesn't have any functional changes, since the output formats will still act exactly the same: formats that employ general contractions will show the blocked contractions, whereas formats employing segmented contractions are already omitting any zero entries.

The main reason for the PR is to improve curation tools: identical basis sets are now detected as identical, since the placing of e.g. free primitives as a dummy part of the contraction pattern vs as one-function shells does not affect how the BSE treats the functions internally. The new convention may also slightly reduce storage costs, since zero entries don't need to be stored.

[codecov-io]

Codecov Report

Merging #199 (285b696) into master (32721c8) will increase coverage by 0.10%.
The diff coverage is 96.66%.