add auto- and cyclic timer

add timer classes useful for measurements and animation control based on older classes already (partially) used in cgsee, cgutils, glwhite (old), cgsandbox, ...

do we need a ```base``` directory?
Why not make ChronoTimer.h public? The class seems to be useful by itself, so why not allow to use it from the outside? I would try not to have too many private classes in general, and only if they're really really internal