Linear MGRIT implementation

[rfalgout]

This PR adds a linear MGRIT implementation to braid. I made it a draft PR because we should change the "switch" to a runtime switch (add a Set function to turn it on/off) and we need to write some brief documentation and probably also a regression test. Otherwise, the code has already been tested thoroughly on the linear branch (this linear-pr branch is the same, but is up to date with master)

[rfalgout]

BTW, when I compile on my mac, I get the following warning:

Building ex-01-pp ...
mpicxx -g -Wall -fPIC -I../braid ex-01-pp.cpp -o ex-01-pp ../braid/libbraid.a -lm -lstdc++
ex-01-pp.cpp:292:4: warning: 'sprintf' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Wdeprecated-declarations]
   sprintf(filename, "%s.%04d.%03d", "ex-01.out", index, rank);
   ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/stdio.h:188:1: note: 'sprintf' has been explicitly marked deprecated here
__deprecated_msg("This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead.")
^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/cdefs.h:215:48: note: expanded from macro '__deprecated_msg'
        #define __deprecated_msg(_msg) __attribute__((__deprecated__(_msg)))
                                                      ^
1 warning generated.

This has something to do with the code merged here from master.

[jbschroder]

My homebrew clang 15.0.2 doesn't throw that warning, but from a little reading, some compilers (like Mac Developer Tools) have started throwing that warning. Either way, snprintf seems to be C99 compliant, so switching to it shouldn't be a problem.