Add more parameter types to `Parameters`

[Mv77]

I have been playing with the core.Parameters class and really like it. One limitation is that it restricts the class of parameters that can be time-varying. This PR adds support for a few more classes: booleans, distributions, and functions.

You can imagine passing time-varying parameters of this class if, for instance you want:

• To indicate whether an agent is retired = True | False for some meaningful change in its income process.
• To have a time varying distribution for income shocks. Maybe this period they are normal, and the next they are lognormal.
• To have a time varying tax function.

I have used this class in my ongoing work and have been very happy with it.

• [x] Tests for new functionality/models or Tests to reproduce the bug-fix in code.
• [x] Updated documentation of features that add new functionality.
• [x] Update CHANGELOG.md with major/minor changes.