MGC SRP comment clarification tweak
MGC tests - SRC_GLOB_WITH_MODE --> SRC_MODE_GLOB
MGC tests - test that destination is not a file if it exists
MGC - this.dest --> this.dest_dir, implemented test for dest-is-not-a-file (points to a test/src file - a bit fragile)
Implemented trailing .{{MODE}}, refactored validator to allow tests for mode's not wrapped in periods
SAME tests - refactored to use new Validator.check_final_result method signature
README.md: Made src mode placeholder format restrictions explicit, added code smells