Labeling of the free parameters in a model

[kbarros]

When fitting a spin model, it is conceptually helpful to track the free parameters. Sunny could make this easier by allowing optional label attachment to each coupling. For example,

set_exchange!(sys, J1, bond; label=:J1)

This would enable quick modification to a model:

get_model_parameter(sys, :J1)         # Initially 1.0
set_model_parameter!(sys, :J1, 2.0)   # sys is updated so that the exchange J1 becomes 2*J1
get_model_parameter(sys, :J1)         # Returns 2.0

This, in turn, would facilitate an interface for future model fitting tools. Updating the strength of an interaction will not need symmetry analysis. This abstraction is also needed for gradient based optimization methods, where the free model parameters must be clearly identified at the System level. See also #292.