Unify constituent minimum values across schemes

When run-time constituents are registered in a scheme with an `instantiate` call, a minimum value is supplied. If the same constituent is registered by multiple schemes and that minimum value differs, the model will error.

Among possibly other options, we could:
1. Create a lookup method that is accessible by any scheme that would enable you to get the "proper" minimum value and set it.
2. Initialize every constituent with min value = 0 and then have a scheme that calls `set_minimum` for each registered constituent.

related to #332

issue noted by @jimmielin in #327