Added the Float and Double precision compilation switch

We can now make the metrosim application for double or single precision
(although currently it does not have any effect). The metrosim.make
makefile will use double precision by default. You must specify the
metrosim_single target to make the program use floats instead of
doubles. The following command will do that for you when in the root
project directory:

make metrosim_single -f metrosim.make