Add multi-source source

[manthey]

This adds a source that can be composited from other sources.

This is still a work-in-progress, needing some features and tests. It should work except for crop and any transforms where s12 or 21 is non-zero or s11 or s22 is less than zero.

There are several ways that the same set of files can be specified. For instance, given a set of eight files named test_orient1.tif, test_orient2.tif, ..., test_orient8.tif, to make a source that uses all of these files, the following yaml files are equivalent:

• (1) explicitly specify each frame and a z value

sources:
  - path: ./test_orient1.tif
    z: 0
  - path: ./test_orient2.tif
    z: 1
  - path: ./test_orient3.tif
    z: 2
  - path: ./test_orient4.tif
    z: 3
  - path: ./test_orient5.tif
    z: 4
  - path: ./test_orient6.tif
    z: 5
  - path: ./test_orient7.tif
    z: 6
  - path: ./test_orient8.tif
    z: 7

• (2) use a regular expression to select frames (they are added alphabetically by c-sort order) and specify a step value

sources:
  - path: .
    pathPattern: 'test_orient[1-8]\.tif'
    zStep: 1

• (3) use a named regular expression to populate the z value. If the name ends in 1, it is assumed to be 1-indexed rather than 0-indexed.

sources:
  - path: .
    pathPattern: 'test_orient(?P<z1>[1-8])\.tif'

[manthey]

Merging this as it works in basic testing and to make it easier to use in other projects.