iFunc: allow cache for value ?

[farhi]

A cache holding the last model evaluation may be added in the class definition, and feval related calls.
It would be returned immediately when called with:

• same parameters
• same axes

[farhi]

We may introduce a private property to hold a cache for the object. This can be used further to store temporary data, hidden. It can be done when converting iFunc to handle based class #184, similarly as done now for idata (with temporary estruct object to be renamed as idata when ready).