Removed old style VIZ_DATA_OUTPUT block format and DX viz mode.

As part of removing the legacy DX format I also got rid of all
related MCell code.
Removed invalid tests.

Several previous tests are now invalid due to removal of old style
VIZ_DATA_OUTPUT block format and DX viz mode. These were removed.
Refactored trimolecular diffusion/reaction code.

Moved much of the trimolecular diffusion/reaction code into
separate source files.
Applied clang-format to all source files.
Removed unused variable.