Allow hyperparameters on geometries

[nabriis]

Is your feature request related to a problem? Please describe.

A clear and concise description of what the problem is. Include code example if possible

import cuqi
dist = Gaussian(0,1, geometry=StepExpansion(n_steps=lambda k:k))

or alternatively

import cuqi
dist = Gaussian(0,1, geometry=lambda k: StepExpansion(n_steps=k))

Describe the solution you'd like

Would be nice to allow conditioning on geometries. Above should return a conditional on k