Define and document the behaviour of libuca virtual class functions

[gabs1234]

Main problem

It is high time we defined the expected behaviour of libuca's virtual class functions. Namely:

• start_recording
• stop_recording
• is_recording
• stopped_recording
• start_readout
• stop_readout
• trigger
• write
• grab
• grab_live
• readout
• set_grab_func
• register_unit
• get_unit
• set_writable
• pspec_set_writable
• is_writable_during_acquisition

I can open a branch in which we can add/specify the behaviour of all of these functions.