Poor time complexity of some algorithm choices

[cerebis]

For larger problems, time complexity makes some more precise algorithms a slow step and potentially impractical once timing estimation is carried out.

These include:

1. optimal leaving ordering in Hclust order module of map_ordering.
2. Jonkers-Volgenant algorithm employed within seriation.

Once the process settles, these could be made optional features.

[cerebis]

For JV, we could simply use it at the end of a convergence run of the faster heuristic method. Currently, the heuristic method is orders of magnitude faster due largely to the implicitly parallel matrix calculations, while JV is known to be a sequential algorithm.

[cerebis]

This issue pertains to unincluded features but is saved for future reference.