Compile with OpenMP

[charnley]

          To compile with OpenMP I edited several lines in `_compile.py` as follows:

COMPILER_FLAGS = ["-O3", "-fopenmp", "-m64", "-march=native", "-fPIC",
                     "-Wno-maybe-uninitialized", "-Wno-unused-function", "-Wno-cpp"]
extra_flags = ["-lgomp", "-lpthread", "-lm", "-ldl"] + COMPILER_FLAGS

flags = ["-L/usr/lib/", "-lblas", "-llapack"] + extra_flags

After compiling the code I compared its performance on a test problem to performance of the qml fork with the same fascf.f90 file. qmllib never seemed to use more than one core, as supported by the timings:
OMP_NUM_THREADS=1:
qml: 0.7533 mins
qmllib: 0.8178 mins
OMP_NUM_THREADS=20:
qml: 0.1390 mins
qmllib: 0.8145 mins

Originally posted by @kvkarandashev in #6 (comment)