Feature precompiled header

[ewanwm]

Use precompiled headers to speed up build times quite a bit.

Tested with and without PCH and got following build times (both ran twice, with a subsequent build done by changing a single file):

--------- without PCH ---------
###### run 1 #######
sys 0m14.775s

###### run 2 #######
sys     0m14.984s
    --- subsequent build (after change in base-matter-solver.hpp):
    sys     0m9.977s

--------- with PCH ---------
###### run 1 #######
sys 0m9.119s

###### run 2 #######
sys     0m9.983s
    --- subsequent build (after change in base-matter-solver.hpp):
    sys     0m4.395s