Implement MI_GENERATE_RESULTS_DIRECTORY to switch on/off MI library output

This implements MI_GENERATE_RESULTS_DIRECTORY to switch on/off file write out of the MI grid database and cross section files (default: on). In addition to the already existing MI_RESULT_DIRECTORY (sets user defined grid database name, overriding the automatically generated one) and MI_RESULT_DIRECTORY_SUFFIX (sets a user defined suffix to the grid database name, useful for systematic variations or other parameter variations, etc.) this allows further options to steer the behaviour of the MI file output.