View for fixed parameters

[MyznikovFD]

No description provided.

[LeonidElkin]

Add an example of using this functionality to the overview after the edits.

[LeonidElkin]

Expected model for this feature:

• After fixing one or more parameters, the result should be a real ParametricFamily subclass.
• The resulting view should expose exactly one parametrization.
• The parametrization used for fixing parameters becomes the view's local base parametrization.
• The view may keep additional metadata about fixed parameters.
• Analytical characteristics should remain available whenever they can be expressed through the
  view parametrization.

In other words, the partial family should behave like a normal one-parametrization
ParametricFamily, with fixed-parameter metadata layered on top.