Optimize part of the quasiparticle environment calculation

[lkdvos]

This optimizes the computation of the environments, especially for the cases where the virtual dimension is large, by computing fewer terms.

In order to do so, the SparseMPOSlice struct now accepts more generic array types, in order to account for slices that are not :.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.83%. Comparing base (f48a5ca) to head (325346e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #138 +/- ## ========================================== + Coverage 82.79% 82.83% +0.03% ========================================== Files 72 72 Lines 4308 4316 +8 ========================================== + Hits 3567 3575 +8 Misses 741 741 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.