Add constexpr to accessors in DenseBase, Quaternions and Translations

Stéphane T.requested to merge
Ste.T/eigen:master into master

Tiny changes which add constexpr to some accessors to DenseBase, Quaternions and Translations.

Reference issue

None: discussed on Discord with chuckyschluz.

What does this implement/fix?

It furthers the liberties of using those classes at compile time, which allows to do pre-computations rather than relying on "magic numbers" to not waste time at run-time.

Additional information

Nothing really.

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