FMMLib tests are skipped on macOS due to lack of OpenMP runtime

The tests that require pyfmmlib are not being run on macOS because the required library libgomp.1.dylib is not being provided. Using conda package on my system I found that libgomp.1.dylib can be provided by libgfortran. In fact, it looks as if the conda-forge version of libgfortran does not provide libgomp, but the defaults version does (??). Oddly enough, there is only a macOS version of libgfortran for conda-forge, so I think Linux may not have this problem.