Refactor FITS writing code

[MatthewWhiting]

The code to save mask, moment map, recon, smoothed etc arrays to FITS files has a bit of repeated code.

It may be useful to refactor into a class-based structure, along the lines of what we did with the reading code (see #166).

[MatthewWhiting]

See #105 - want to include baseline array as well here.

[MatthewWhiting]

Starting this in [1115],[1116]. Idea is to have a base class that holds all necesary members and does basic creation, header writing and closing, with derived classes for each type of array that do specific header writing and writing of data.

The code should all go in FitsIO at the end (currently in Cubes).

Will also need a Cube method to handle the calling of each of them - this will simplify the interface as well (ie. from mainDuchamp.cc's perspective).

[MatthewWhiting]

Done and implemented in [1120],[1121]. Closing ticket.