Add Hilbert space filling curves and other changes

Added Hilbert Space Filling Curve and made several changes to Space Filling Curve load balance methods, add new timers, fix some output to reflect changes to input, and general cleanup.

Update global variables for Clang based compilers

This update allows the code to be compiled with Clang based compilers. When global variables are being used with the Clang based compilers, they can only be declared in one source file and have to be "extern" to other source files.

Bug fix

Fix longstanding bug in block.c by decrementing the number of parent blocks when we recombine a set of child blocks.

Update due to new C standard

The new C standard has introduced a math function called beta, which conflicts with a variable in the code, so it got changed.

v1.6.4: Update stencil.c

Correct a couple of errors that show up at scale and fix formatting of some print statements.

v1.6.3: Update rcb.c

Correct a bad implementation of the "group_blocks" option.

v1.6.2: Update sfc.c

This release includes three bug fixes in the space filling curve load balancing code. Redo the release again so sfc.c will compile and the version number in profile.c has been incremented.

v1.6.1: Update main.c

A couple of corrections that affect the use of simulation time.

Add Space Filling Curve

Add Space Filling Curve as a load balance option. Clean up some typos in some files as well as getting rid of some left over code from deleted options. Rework some include files to keep variables in a more consistent order.

v1.5.0: Update profile.c

Added options for load balancing to better compare to codes, added an option to send one face at a time, corrected some MPI inconsistencies, and deleted the target code.