Rename the template defintions to end in hxx

[peterfpeterson]

This is a more standard extension than tcc as described in https://stackoverflow.com/questions/5612791/c-template-and-header-files. It also changes a doxygen string now that doxygen recognizes the filetype. The .gitignore was edited to discourage people from using the .tcc extension.

There is no associated issue.

To test:

If it compiles, it works.

This does not require release notes because it is a very internal change to aid compilers and IDEs understand the file's purpose better.

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[rboston628]

All changes are obvious, except the doxygen changes. Those look good, too.