Split mechanisms from fvm

[halfflat]

We have mechanism-handling code and fvm-specific code, and there is no real need for these to be as intimately tied as they currently are. The required dependencies between them are opaque and a bit confusing due to our source layout and class and file naming conventions in this area.

• Give unto fvm what pertains to the finite volume method discretization, and give unto mechanism that which belongeth to the mechanism code.
• Rejig file paths, names to suit.
• Make public headers to enable out-of-tree mechanism builds (this might require a small measure of refactoring).

[halfflat]

Part of general source mangling task #98.

[thorstenhater]

All related issues are closed, so this can be closed as well.