Add .clang-format

[gtkiku]

• Constructed to roughly match existing coding style, not an exact match unfortunately due to somewhat inconsistent previous usage throughout codebase.

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On a sidenote, CONTRIBUTING.md refers to Stroustrup style, but that doesn't seem to apply. Stroustrup uses a variant of K&R, and for example opening braces are placed on a new line, whereas the current codebase keeps them on the same line (generally speaking at least, grep -r '^{' finds some exceptions).

[pjaaskel]

Thanks. Can you update the CONTRIBUTING.md as well? Early in the project we did manual code reviews and watched the style closer, it is indeed not exactly Stroustrup style. For the record, I do not have strong preferences to any particular style, but it's good to converge to unified style in each code base.

[gtkiku]

Rewrote the TCE coding style section, it now references .clang-format and also shows an example of using it to just format new modifications so as to not cause too much splash damage to files that don't fully abide by it.