[WIP] Implementation of absorbing boundary layer

[cjvogl]

This is still under development but there seems to be enough interest in using the absorbing boundary layer to open a PR.

The current approach to implementing the absorbing boundary layer is

1. the user specifies parameters needed for the absorbing boundary layer in setrun.py
2. those parameters are then both written to abl.data as well as used to increase the computational domain and update other parameters that are written to claw.data

My developmental to-do list is

• implement the ability for the user to specify a relative error in setrun.py that is then used to determine the abl depth, if the trigonometric mapping is selected