kdaectrl: fill out more of nexus_structure in run start

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, https://github.com/ISISComputingGroup/DataStreaming/issues/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.