[cover] Improvements for small or homogeneous data

[cover] Improvements for small or homogeneous data

* The algorithm would bail as soon as it found one epoch that
  contained no new segments. Change it so it now has to fail
  >= 10 times in a row (10 for fastcover, 10-100 for cover).
* The algorithm uses the `maxDict` size to decide the epoch size.
  When this size is absurdly large, it causes tiny epochs. Lower
  bound the epoch size at 10x the segment size, and warn the user
  that their training set is too small.

Fixes #1554