updates pml routines; adds pml array permutations to be usable with mesh coloring
updates make rules in directory src/inverse_problem/ to avoid overlaps for make clean
small code beautify
uses local arrays for PML_dpotential_dxl,.. for slightly faster acoustic PML computations
updates deville routines to work with NGLLX == 5, 6 or 7
added doc/definition_of_unit_testing_and_regression_testing.txt