PCell update: Library#refresh should call coerce_parameters_impl

[klayoutmatthias]

Rationale: the "refresh" function is used to update the PCell layouts to the "current version". When the PCell implementation has changed, coerce_parameters_impl needs to be called, because the parameters may no longer be valid or need to be re-evaluated.

Right now it appears that "refresh" does not invoke "coerce_parameters_impl".

[klayoutmatthias]

The solution is implemented through PR #1083.

Please note the following: coerce_parameters_impl now is called also prior to "produce" to ensure that produce will always receive consistent parameters. When coerce_parameters_impl raises an exception, the PCell will display the error text rather than producing layout. However, when coerce_parameters_impl changes a parameter during that phase, the new value is not persisted.

In addition, coerce_parameters_impl is also called during "Library#refresh" as the ticket requests. In this case, changing the parameter value in coerce_parameters_impl will change it persistently.