Add PRT support

[dbrakenhoff]

First attempt at addressing #390. Still somewhat under development on the flopy side, so creating a draft PR for now.

Adds

• PRT submodule with all relevant packages (except FMI for now)
• EMS solver
• improve some of the modpath utilities to be applicable for PRT models (i.e. don't need a modpath model).

Todo:

• add nlmod FMI package?
• utilities to write PRP data (without having to use modpath tools which currently need to perform lots of coordinate transformations)
• figure out how to specify backward tracking, add example
• Make notebook slightly more interesting, e.g. forward and backward tracking
• showcase PRT features, e.g. pulsing particles?

[dbrakenhoff]

Requires latest develop version of flopy (or maybe even one of the open PRT PRs) to work.