Implement absorbing boundary conditions in main branch

[kylegodbey]

As the title suggests, it would be nice to have the absorbing boundary conditions in the main line of the software. I talked about it with @aaron-philip and he will check the feasibility and report back.

[pdstevenson]

That would be great. I agree it should be in the main branch. Is the idea to put the static imaginary potentials in the main branch, or TABC, or both?