Made some changes to allow program to compile with gcc.
Removed domain distribution types.
Added test target as dependancy of run_test in Makefile. Rearranged order of dependencied for grav_force.test and lj_force.test in test/Makefile to ensure abstract_distribution_type.mod exists before they're compiled.
Changed interface for pair_operation for distribution types.
Removed domain distributed distribution entirely.
Made all necessary changes to force modules and executables for them to compile and run as expected.