Change how plankton dynamics is set

[gustavdelius]

Currently the function that is used to project the plankton abundance forward by one time step is stored in the @plankton_dynamics slot of the MizerParams object as a function. This leads to problems when the params object is shared with others, see #91 . We should therefore change to saving the name of the function instead of the function itself. Presumably it would then also be a good idea to change the plankton_dynamics argument to the setPlankton() function to a string rather than a function.