Remove methods get/setCustomParameters from ImageGenerator interface

[MStefko]

Interfaces should be implementation-agnostic, i.e. you shouldn't care which specific implementation of the interface you are talking to when you use one of the interface's methods.

setCustomParameters(Map map) obviously breaks this pattern, since you need to know what are the keys that the class expects to find inside map.

getCustomParameters() could in theory be kept, since you could iterate over the returned map to e.g. print all parameters of the simulation, but that is again skimming the implementation-specific line.

These two methods are not required for the interface to work and should be removed.

[kmdouglass]

Note that ImageGenerator was renamed to Simulator in #67