Phate

Changeset:

* reinstate `use.alpha` parameter for Seurat compatibility.

Changeset:
* updated parameters `k -> knn, alpha -> decay` to match Python version
* better handling of load errors
* added FAQ to README

Changeset:

* add `cluster` submodule
* adds `cluster.kmeans` function to perform KMeans on the diffusion potential
* add `dpi` argument to `plot.rotate_scatter3d`

Changeset:
* phate.plot.scatter now accepts a list of matplotlib colors
* phate.plot.scatter now returns the axis on which the plot was drawn
* minor logging changes

Changes:

* Use `tasklogger` for logging; fixes issues with Windows 7 verbose output
* Use `scprep` in tutorials for simpler analysis
* Change default value of alpha to 40 for faster computation of kernel
* `phate.plot.scatter` now takes keyword arguments `x`, `y` and `z` rather than a list-like
* `phate.plot.scatter` has keyword arguments for axis label prefix `label_prefix` (as an alternative to labelling axes individually), plot title `title`, and legend location `legend_loc`
* minor bugfixes