Add TACSMeshReader support for 16 and 25 node quad elements

[A-CGray]

Adds the ability for TACSMeshReader to read in higher order quad elements. I also updated the annulus example to work with these element types, results below.

Unfortunately, because CQUAD16 and CQUAD25 are not NASTRAN element types, the pymeshloader cannot read these elements.

Additionally, I was running into some multiple definition errors caused by a couple of variables that are defined in the TACSMeshLoader header file when compiling code that links to both TACS and an object file that uses TACSMeshLoader, for example:

TACSHelpers.h:

#pragma once

#include "TACSMeshLoader.h"

void createTACSAssembler(const char *filename, TACSAssembler *&assembler);

TACSHelpers.cpp:

#include "TACSHelpers.h"

void createTACSAssembler(const char *filename, TACSAssembler *&assembler) {
  // Create the mesh loader object on MPI_COMM_WORLD. The
  // TACSAssembler object will be created on the same comm
  TACSMeshLoader *const mesh = new TACSMeshLoader(MPI_COMM_WORLD);
  mesh->incref();
  .
  .
  .
}

RunTacs.cpp:

#include "TACSHelpers.h"

int main(int argc, char *argv[]) {
  .
  .
  .
  createTACSAssembler(argv[1], assembler);
  .
  .
  .
}

If you first compile TACSHelpers.o, then compile RunTacs against both TACS and TACSHelpers.o, you'll get a linking error like this:

/usr/bin/ld: /tmp/ccazYb9Z.o:/home/ali/repos/tacs/src/io/TACSMeshLoader.h:25: multiple definition of `TacsMeshLoaderElementTypes'; TACSHelpers.o (symbol from plugin):(.text+0x0): first defined here
collect2: error: ld returned 1 exit status

This is because TacsMeshLoaderElementTypes is defined in TACSMeshLoader.h but outside the TACSMeshLoader class and thus becomes defined in TACSHelpers.o, when you then compile RunTACS, it sees this definition and the original one from TACS.

Since these variables are only ever used inside the mesh loader class, I just made these variables static const attributes of TACSMeshLoader, which fixes this issue.