Complex function/functionspace utilities

[JHopeCollins]

The complex blocks are implemented as a 2x2 system of real blocks. There are a few different operations related to this in DiagFFTPC that could be factored out into their own module which can be tested, reduces rewriting code etc. Possible things to include:

• Creating the complex function space from the real function space (currently done in the initialize method)
• Transferring boundary conditions from the real function space to the complex function space (currently in set_CblockV_bcs method)
• Copying between two real functions and the real and complex components of a complex function (currently used in update and apply methods)
• Building block solvers and forms for the complex space from forms on the real space (currently in initialize method)
• Anything else?

[JHopeCollins]

This is being worked on here: https://github.com/JHopeCollins/complex-proxy

[JHopeCollins]

The complex_proxy module is now in a reasonably complete state, with submodules for using either VectorFunctionSpace or MixedFunctionSpace as the complex space. PR #109 refactors DiagFFTPC to use this module.