Domain and System Resizing Utilities

[martintb]

There are two, somewhat separate ways that users might want to 'resize' their system:

1. The user want's to change the dr or length of their pyPRISM.Domain or to interpolate their correlations functions to a new dr or length.

2. The user wants to 'coarsen' their system down to a smaller number of site-types. For example, a 4-component system consisting of two copolymers might be coarsened down to a two-component system by combining correlation functions.

Both of these features would be useful utilities and should be somewhat straightforward to implement.