Revert draw functions: alter parameters only

[twallema]

• I have checked to ensure there aren't other open Pull Requests for the same update/change
• I have updated the documentation accordingly.

Describe your fixes/additions/changes

Following #99, altering the initial condition thru the draw functions has become redundant. Draw functions now have their original format,

draw_function(parameters, par_1, par_2):
    # make alteration to parameter dictionary
   return parameters

where par_1 and par_2 have to be provided to the sim() function thru the draw_function_kwargs. To alter an initial condition, use an initial condition function (see PR #99) with arguments. Because these arguments become model parameters, they can be altered through the draw function.