Clean-up interface and prepare for span interface-changes

This MR removes the real_t type, as it mainly makes the interface less transparent. If we were ever to change the type of calculations to e.g. single precision, we wouldn't want to do this globally for every number ever used in everybeam, as e.g. time and frequency values have to stay a double.

This MR also contains some preparation for !385 (merged), such that MR can focus on interface changes only, which is a big change by itself already.