Clean up --help

[weinbe2]

Some ideas:

• Add support for --help [option group] to minimize the amount output by --help (or some other appropriate mechanism, whatever CLI11 makes easiest
• Improve the granularity of option groups, for ex clover specific, twisted specific, etc, similar to how there are already groups for eigensolves and MG. This would be a follow-up to the above suggestion of group-specific --help
• Potentially update CLI11, which is non-trivial because of some hacks to support MG flags (putting --mg-something [level] [value] 's value into the level element on an array of inputs)