Add Triple Point Shock Problem as Euler Application

[DamynChipman]

Adds an additional example under applications/clawpack/euler/2d/triple. This runs the triple point shock problem as outlined here. The implementation has been verified.

[DamynChipman]

I failed to add the regression test files, I added them in the latest commit.

[DamynChipman]

The only test that fails is the CMake build on MacOS. It fails when running the regression test for triple. It looks like it fails when trying to write an output file:

...
[fclaw] Output Frame    2  at time   2.00000000e-01

[libsc 1] Caught signal SEGV
[libsc 1] Abort: Obtained 7 stack frames
[libsc 1] Stack 0: 0   triple                              0x0000000102364e18 sc_abort_handler + 56
[libsc 1] Stack 1: 1   ???                                 0x00007ffeed9946b0 0x0 + 140732884666032
[libsc 1] Stack 2: 2   libmpi.40.dylib                     0x000000010253386c mca_fcoll_vulcan_file_write_all + 3297
[libsc 1] Stack 3: 3   libmpi.40.dylib                     0x000000010256d76d mca_io_ompio_file_write_all + 69
[libsc 1] Stack 4: 4   libmpi.40.dylib                     0x000000010249caf9 MPI_File_write_all + 196
[libsc 1] Stack 5: 5   triple                              0x00000001022a9e77 fclaw2d_vtk_write_field + 455
[libsc 1] Stack 6: 6   ???                                 0x0000005100000005 0x0 + 347892350981
--------------------------------------------------------------------------
MPI_ABORT was invoked on rank 1 in communicator MPI_COMM_WORLD
  Proc: [[12403,1],1]
  Errorcode: 1

NOTE: invoking MPI_ABORT causes Open MPI to kill all MPI processes.
You may or may not see output from other processes, depending on
exactly when Open MPI kills them.
--------------------------------------------------------------------------
...

I'll remove output for the regression tests to see if that fixes it.

[DamynChipman]

Merged recent develop commits into feature-triple. Assuming they pass units tests, this feature will be ready to merge into develop.

[DamynChipman]

Ready to merge!

[scottaiton]

I updated the Automake files for you. I'll let @donnaaboise look at this and merge.