data_specification_loaded to turn off requires data spec / reset

[Christian-B]

This pr changes how requires_data_generation is used.

Master

Things that turn requires_data_generation on:

• setup
• hard reset
• View.set_requires_data_generation()
  -- Called by vertex if synapse_dynamics.changes_during_run
• View.set_requires_mapping
  -- called by anything that adds to the graph

Things that turned requires_data_generation

• Finish_run

CHANGE

data_specification_loaded to turn requires_data_generation off
writer.set_requires_data_generation() (which can be called during run)

USAGE

If we allow timed runs to be stopped this allows requires_data_generation to be turned on in a way that is kept when Finish_run is called.

REQIORES

SpiNNakerManchester/SpiNNFrontEndCommon#1227

• requires checking get_requires_data_generation if in a second non reset run
• calling data_specification_loaded after loading / reloading