Move fesc to a transform

[christopherlovell]

Currently fesc is a hard coded argument on a spectra generation, but in an abstract sense applying an escape fraction is just a scalar transformation of an array. We should change all fesc instances to an emission model Transform derived class. This will also allow the implementation of an fesc - 1 method, for obtaining the escaped and transmitted spectra, for example.

Will lead to breaking changes to all methods that currently use the fesc argument, but more flexibility going forward.