f_systime improvements

[Necktschnagge]

what?

I created a new f_systime.h/cpp lib that takes care of starting / stopping timer on RTC oscillator and provides a class for a system-wide time.

already done:

• created files with an concept explanation in math notation
• created pseudo algorithm in math notation
• (afterward) created a class with nearly all described features, some are only declared for later implementation
• made a little doc for u-programmer, who want to use this library quickly without reading everything

to do next

• make fps
• look through for better descriptions
• look through for consistence and correctness

[Necktschnagge]

found dependency

we need some += or at least + operator from time::EMT

[Necktschnagge]

• operator is there, I must have been a misunderstanding

[Necktschnagge]

done:

• one time check of correctness and consistence
• made FPS
• lib reached RC state

to do:

• anything at fps notes.
• especially second whole check, enhanced descriptions, tests, ....