Allow standalone system simulation component

[rosesyrett]

This involves changing the SystemSimulation input parameters, and making it so that the slave scheduler runs an 'initial' tick in the same way as master.

Relates to https://github.com/dls-controls/tickit/issues/149

[codecov[bot]]

Codecov Report

Merging #155 (d6cd04f) into master (0ff34b1) will decrease coverage by 0.35%. The diff coverage is 68.75%.

@@            Coverage Diff             @@
##           master     #155      +/-   ##
==========================================
- Coverage   94.69%   94.35%   -0.35%     
==========================================
  Files          44       44              
  Lines        1264     1276      +12     
==========================================
+ Hits         1197     1204       +7     
- Misses         67       72       +5     

Impacted Files	Coverage Δ
src/tickit/core/management/schedulers/slave.py	91.93% <66.66%> (-8.07%)	:arrow_down:
src/tickit/core/components/system_simulation.py	97.67% <100.00%> (-0.11%)	:arrow_down: