Enable change of `w_pp_cutoff` parameter in an existing model

[gustavdelius]

While setResource() has a w_pp_cutoff parameter, supplying this does not actually change the cutoff in the resource carrying capacity unless the carrying_capacity argument is also supplied. Furthermore, when one tries to set the cutoff to a smaller value, then one gets an error because that would lead to a carrying capacity that is not everywhere larger than the current resource abundance. setResource() should therefore also cut off the current resource abundance at the new w_pp_cutoff.

[gustavdelius]

I think that if w_pp_cutoff is provided to setResource() but carrying_capacity is not, and if the new cutoff is smaller than the previous cutoff, then the function should cut off both the initial resource abundance and the carrying capacity at the new cutoff. If the new cutoff is larger than the existing cutoff, then an error should be issued because there is no way to guess the appropriate value for the extra carrying capacity.