time tagger updates

[meeg]

Work in progress to make the time tagger (still in testing) more usable, though some of the changes are of more general effect.

non-breaking changes:

• bugfixes and tweaks to the time-tagger firmware block (remove jitter on arm input, make it possible for the tProc to read the size of its tag queue)
• clean up the time-tagger driver
• add some code for ADC calibration to the RFDC driver

possible breaking changes for tproc v2 users:

• change how loops work, so the loop variable goes from 0 to N-1 like in a typical for loop (instead of from N to 1) - this will affect programs where the loop variable is used as an array index or similar
• it's now possible to specify multiple generators in add_pulse(); some checking will be done to make sure all the specified generators can use the same pulse definition, and pulse() enforces that you can only play a pulse on its specified generators - this will affect programs where a pulse was defined for one generator and played on others