Icepool

* Faster algorithm for keeping the single highest/lowest.
* Fix `from_sweights()`.

* Fix `max()` and `min()` casting to dice.

* Capitalize `Die()` and `Pool()` factory methods.
* `PoolEval` renamed to `EvalPool`.
* Implemented `min_outcomes` for pools. Direction is now determined by `EvalPool`.
* Allow dice with zero weights or zero outcomes.
* `BaseDie` subclasses are now `ScalarDie` and `VectorDie`.
* `==` and `!=` compare outcomes again. Dice are no longer hashable by the standard `hash()`.
* Rewored `Pool()` arguments.
* Added several methods.

* Fix `weights_le()` etc.
* `set_outcomes()` is now public.

* `min_outcome()`, `max_outcome()` now accept multiple dice.
* Fix slice range going below zero.
* `num_dice` is now an optional parameter for `keep*()` and `pool()`. Exactly one out of `num_dice`, `min_outcomes`, and `max_outcomes` should be provided.
* Removed right-side cast for `` operator, as it leads to confusion with either other operator casting or `d()` casting.
* Replaced key-value sequence option for `die()` with a sequence of weight-1 outcomes.
* Add `has_zero_weights()` method to dice.
* `align()`, `align_range()` are now public.

* Remove numpy install dependency.