get_default_options for multiple subsolvers a la -fieldsplit

[JHopeCollins]

This is a convenience function to extract default arguments when you have multiple subsolvers whose prefixes have a common base, e.g. using -fieldsplit_option value to set defaults for both -fieldsplit_0_option and -fieldsplit_1_option

[JHopeCollins]

I think that this is ready to go now.
To use, you create a DefaultOptionsSet with a base_prefix and a set of custom_prefix_endings that gets passed to the OptionsManager. The OptionsManager checks that its own options_prefix is one of those in the DefaultOptionsSet. Any default options have the lowest priority, i.e. they are overridden by either the parameters passed to the OptionsManager or anything on the command line.
Docstring is quite long but hopefully clear.

Also I did a bit of docstring wrangling so things are linked rather than just rendered as code.

[connorjward]

In general I'm very happy. I think this is well written and the right approach.