refactor stylefiles

According to the beamer-user-guide sections 16.1 (Inner Themes) and 16.2 (Outer Themes), the inner theme should "install templates for elements that are typically “inside the main text,” while the outer theme should "install templates for elements“ around the main text.

The way it appears to me now is that beamerouterthemetuhh.sty and beamerinnerthemetuhh.sty fulfill the same purpose in the flavor of an outer theme as in the beamer user guide.

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Additional todos:

• Remove \inst and \and functionality for commands other than \author and \institute.