Add [sg]etter methods for optimizer params
Add documentation for class and [sg]etters
Remove `virtual` keyword from [sg]etters

These setters and getters are general enough that they shouldn't need to be
overridden by any subclass
Set defaults for optimizer params
Add defaults to documentation
Refactor GslOptimizer to use getters for params
Make optimizer parameters protected so subclasses can use directly
Pass GslOptimizer as the context to the GSL level

This is done so we can access parameters of the optimizer from the non-memnber
functions GSL needs.
Merge pull request #269 from dmcdougall/minimizer-options

Implement [sg]etter methods for optimizer parameters