improvements needed to Masks documentation

[amoodie]

The Mask objects are complicated and it is not well documented.

Areas to be improved:

There are currently some pieces of this here and here.

• The first part of the latter should probably also be described in the former (a list of masks is a common way we do this now).
• There should be some description in the Guide about the attributes and methods of the Mask; especially .trim_mask(), .mask, and .integer_mask.
• The guide should include an example of cleaning up a Mask using simple scipy operations, explaining that any workflow that works for your data can be substituted here.