ExtractChannel and FlatFieldDatasource 5D / multichannel datasource fixes

[barentine]

Addresses issues:

• ExtractChannel._pickChannel uses legacy image.data access and collapses the wrong index if you pass it a XYZTC shape [1024, 1024, 1, 1, 4] datasource and try to pick out one of the 4 channels
• FlatFieldDataSource doesn't preserve parent datasource t/z info in its current state, and can also mess up channel extraction.

Is this a bugfix or an enhancement?
bugfix
Proposed changes:

• use image.data_xyztc access in ExtractChannel module
• add units tests for ExtractChannel
• store parent datasource t/z info in FlatFieldDataSource

FlatfieldDataSource currently inherits from XYTCDataSource, i.e. the old-style datasource base. Certainly it would be better to do a full re-jig, but this was the fastest fix for me.

Checklist:
The below is a list of things what will be considered when reviewing PRs. It is not prescriptive, and does not
imply that PRs which touch any of these will be rejected but gives a rough indication of where there is a potential
for hangups (i.e. factors which could turn a 5 min review into a half hour or longer and shunt it to the bottom
of the TODO list).

• Does the PR avoid variable renaming in existing code, whitespace changes, and other forms of tidying? [There is a place for code tidying, but it makes reviewing
  much simpler if this is kept separate from functional changes. The auto-formatting performed by some editors is particulaly egregious and can lead to files with thousands
  of non-functional changes with a few functional changes scattered amoungst them]

If an enhancement (or non-trivial bugfix):

• Has this been discussed in advance (feature request, PR proposal, email, or direct conversation)?
• Does this change how users interact with the software? How will these changes be communicated?
• Does this maintain backwards compatibility with old data?
• Does this change the required dependencies?
• Are there any other side effects of the change?