Make headers reusable

There are a lot of function definitions in the headers (example: solver.h).

If we split some binary into multilpe compilation units using the same header, this will result in duplicate symbols.

Therefore: Move stuff out of the headers into cc files.

Directories still to do:

• common
• geometry
• input
• lattice
• materials
• mesh
• neighbourhoods
• orq
• preprocessor
• solver