FEATURE: support linked parametisation so mech prop can share parameterisations

[rob-hamill]

so would do something like this:

        spatial_parameterisations={
            "elastic_modulus": KnownSpatialParameterisation(),
            "poissons_ratio": KnownSpatialParameterisation(),
            "yield_strength": SliceWiseSpatialParameterisation(slice_partition),
            "hardening_modulus": LinkedSpatialParameterisation(linked_parameter='yield_strength',free_dof='value'),
        },

current:
metrics=[
SliceWiseForceReconstructionMetric(slice_partition),
],

        optimiser=SliceWiseIndependentLeastSquares(),

for linked param:
metrics=[
SliceWiseForceReconstructionMetric(???),
],