Find good math font sizes

Currently the font size for math equations is not set properly (or at all).

We need to find good math font sizes for different base font sizes. e.g.:
text font = 20pt -> math font = 18pt
text font = 24pt -> math font = 22pt

The math font size can be set using
\DeclareMathSizes {t-size} {mt-size} {s-size} {ss-size}
where mt-size is the math font size to be used when the t-size is used for text font.
Additional script-size (e.g. powers) and script-script-size (e.g. powers of powers) have to be specified.
See here.

I propose to set these values in beamerfontthemetuhh.sty.