Added a template approach to specify imaging and self-calibration settings

[tjgalvin]

To-date imaging (e.g. wsclean), self-calibration (e.g. gaincal) and other things have been hard-coded into the flint codebase. When updating these throughout the rounds of self-calibration a dictionary with updated values would be obtained and splatted into the creation of the appropriate Options.

This is pretty tiresome. If the git history is examined it would be pretty obvious how many times commits were made purely to change the image size, solution interval, or something trivial.

This pull request introduces a template yaml file that is used to set the options that may or may not evolve as the pipeline runs. The idea is that a file (at the moment a yaml file) is parsed into a Strategy class, which is nothing more than a glorified name for a dictionary (but nice for type checking). Throughout the pipeline the Strategy with be investigated to:

• get defaults for an Option mode
• check for and updated values for the specific round of imaging
• update the defaults
• return the updated defaults

The existing infrastructure to overload the hardcoded defaults of an Option type class is used as normal.

[tjgalvin]

This addresses #4