kdaectrl: fill out more of nexus_structure in run start

[rerpha]

in a run start we currently just add placeholders for blocks and for events.

we should write some static fields (based off KDAECTRL's PV values), these are the easy ones:

title - based on TITLE (assume this won't change and just do it static for now, #44 needs to be implemented for this)
start_time - based on start time of message ( we already put this in the run start)
name - the instrument name
run_number - the run number
beamline - the instrument name
program_name - probably shouldn't be ISISICP

the /instrument group
which contains

• name - the instrument name
• source (a group) containing:
  • name: ISIS
  • probe: neutrons
  • type: Pulsed Neutron Source

potentially more challenging:
run_cycle - the current cycle. how does the ICP get this?
experiment_identifier - not sure what this is

as part of this ticket we should actually document these fields in the dev wiki.
todo: there are also some fields which can only be written at the end, write tickets for these.