Implement a mechanism for producing masks based on simple image contrast

[WilliamJudge94]

Using the O K-edge with some ptychography data produces masks that are inverted (maybe?) from what they should be (i.e. 1's mark active material instead of background). We need to review the various edge combinations to make sure they're at least all consistent.

Also, there should be a page in the docs about using masks.

[canismarko]

Turns out this is the result of the underlying data, the edge jump happens to be relatively high in the background. This motivated the development of a second form of masking, named (contrast mask). @WilliamJudge94 is developing this.

[canismarko]

Code was completed with 0646af2, but the documentation needs a little cleaning up. 40f9779 added documentation on how to use the contrast mask, and lists one drawback of the absorption edge masking approach. We should list the benefits of XAS edge masking and the benefits and drawbacks of contrast masking.