Implement "two-pass" version of rise/set/transit functions

This is prompted by the discussion around these functions in #424 - specifically https://github.com/astropy/astroplan/pull/424#issuecomment-543156136 and the related thread.

@StuartLittlefair pointed out that a two-pass solution, where you can get away with a finer grid by doing it once to get a 24-hr guess, and then re-gridding around that to do a better guess. That should let the user get away with a much smaller `N` parameter (from #424), and make things quite a bit faster as a result.

Start with the rise, set, and transit functions and read the discussion in issue #424, especially the linked pull-request comment and related thread. Determine how the proposed two-pass approach should use a coarse 24-hour estimate and re-grid around it; done means supporting a smaller N while producing a faster, more accurate result.

Written by the indexing model from the issue text.