Using the OpenMP library

[tkoskela]

In order to write the number of threads being used into the output file, I have to call omp_get_max_threads from the omp library. I've put the call in a separate module. However, compiling the module will fail if -fopenmp is not included in compiler flags because it won't be able to use omp_lib. We discussed a few options for making this work flexibly

1. Using a preprocessor directive to only compile the module when compiling with -fopenmp
2. Creating a dummy omp_lib.mod that would get included when compiling without -fopenmp. For example https://people.sc.fsu.edu/~jburkardt/f77_src/openmp_stubs/openmp_stubs.html
3. Not calling omp_get_max_threads, but using the value of the environment variable OMP_NUM_THREADS instead

[tkoskela]

I've implemented a simplified version of option 2. in #195 I created a dummy version of omp_module.f90 and added a Makefile variable in system.make that compiles the dummy version instead of the real one when set.