Heatmap slicer improvements

[ianhi]

Meta issue to keep track of the various potential improvements to hyperslicer raised in:
#57
#33
#29

as those issues have significant overlap.

29: allow arbitrary angles of heatmap slicer
e.g. heatmap_slicer(arr1, arr2, slices = [15, 37]) for slices at 15 and 37 degrees.

33: truly arbitrary slices of heatmaps
with a draggable line on the image following some of the examples in: https://stackoverflow.com/questions/7878398/how-to-extract-an-arbitrary-line-of-values-from-a-numpy-array

(I'm less stoked about this one currently - seems hard to do well and not like a huge value add)

57:

Currently slices accepts: 'vertical', 'horizontal', and 'both'. This has potential to become a nightmare of special casing once arbitrary angles are supported. In particular it should probably not accept 'both' and instead you use a list to specify.

or at the very least both should be renamed to something more descriptive like cross hairs or something that will still have semantic meaning when arbitrary angles are supported