Standardize component constructor params/param_defaults/kwargs

[kmantel]

Parameter values in component constructors can be given either as standard keyword arguments, or in dictionary form. For shell/base classes and Compositions, the argument for the dictionary form is param_defaults, and for most other components, it's params. I think we should decide to either use params or param_defaults globally or drop the extra dictionary specification and assume that using a **params syntax would be enough for that situation.

If we keep it, I think we should make sure that it's clear in the documentation that those values silently take priority over values passed as keyword arguments or add a warning if an override occurs.