* A variable (IDENTIFIER) was added to allow the 'identifier' target to
  search source code and header files for a matching hash parametrically.
* The phony 'identifier' target greps for the IDENTIFIER pattern in each file
  and fails if any source/header doesn't contain the pattern.
* The identifier target is added as a dependency to fullsubmit and
  partialsubmit.
* Editing IDENTIFIER is now part of the first student TODO in the Makefile.

Added compiler version 7.1.0 before 6.2.0, just in case CAEN and autograder don't both have the same version available.  If both exist, should default to 7.1.0 (the first one in each path variable).

New target 'mac2caen' relies on SSH multiplexing, and an open SSH session
to keep files synced FROM Mac TO CAEN.

Slightly edited these targets (release, debug, all) for consistency.
Also picked some nits in comments and 'make help'.

* Previously we were checking all CPP files, we don't need to check
test drivers (test*cpp) for style.
* Instead of *.h*, use a more correct *.h *.hpp

Changed compile flag from -std=c++17 to -std=c++1z for Mac users; still works on CAEN and autograder.