gclean should provide image/mask information

[schiebel]

To support multifield imaging where the imaging parameters could be in an outlier file or other locations, gclean should provide a member function (or functions) which allow for getting information needed for the GUI. The things I can think of now are:

• path to residual
• path to mask
• initial values for all iteration etc. parameters for each image (maybe this is already available in the record returned by the next/anext functions?)

[Kitchi]

Copying over @schiebel's comment from #47 :

@Kitchi the latest commit to casagui contains the reorganized GUI to support multiple fields. It should pass automask parameters to gclean.update. Currently it uses separate gclean objects to generate masks and to generate and update residuals for each field. This was done to allow for testing without the implementation of multifield support within gclean and hopefully done in a way that will make it easy to remove the extra gclean objects in the future. At the point, interactive clean will depend on a single gclean object to handle the multiple fields. However, doing this depends on support in gclean for:

parameter validation in __init__
access to paths to mask images
access to paths to residual images
access to initial values for automask parameters (displayed in GUI)
access to initial values for iteration parameters (displayed in GUI)

I can't think of other things that may be needed, but basically because the configuration comes from bespoke description files for outliers it makes the most sense for gclean to ingest parameters and the provide accessors needed for the initial configuration of the GUI elements (instead of now where InteractiveClean processes the parameters and gclean processes them separately).

A mustache template used to generate _gclean.py from tclean.xml should make it easy to validate the parameters passed to gclean.init and keep gclean in sync as tclean changes.