Release v0.6.1

* Fixes ``P.total`` to return ``#!python 1`` for empty pools, consistent with the empty product (see [this
        explanation](https://www.johndcook.com/blog/2015/04/14/empty-sum-product/)).
* Renames ``dyce.evaluation._LimitT`` to ``dyce.evaluation.LimitT``.
* Fixes issues pertaining to histograms with zero totals after allowing outcomes with zero counts in non-normalized ``H``.
* Fixes issue that would lead to incorrect results when making certain arbitrary selections with heterogeneous pools (e.g., with ``P.rolls_with_counts``).