Is your feature request related to a problem? Please describe.
No

Describe the solution you'd like
Currently, the optimisation variables are defined in the optimisation_problem.py file with everything else.

It would be good to move the definition of the variables out into their own module to reduce the complexity of the optimisation engine.

Describe alternatives you've considered
None

Additional context
Just move the variables into their own module. Could potentially create sub-modules for each of the types of variables e.g.:

variables /
   electricity_flow_variables /
       renewables.py
       battery.py
....

Blocked By
#3