Recompute proposal when acquisition function is changed

[kiudee]

It is possible for users to change the acquisition function manually during an optimization run.

Example:

from bask import Optimizer
from bask.acquisition import PVRS
opt = Optimizer(...)
opt.acq_func = PVRS()

This however only takes effect after the next point has been evaluated. It would be useful if we implement a setter which does the recomputation.