Change how min/max/default values for parameters are managed.

Make min/max values strings, and test-convert them along with the
default value during startup, to see that we can.  At the same time
normalize the strings with the parameters tweak function.

List min/max values in .rst doc.

param.show will flag default values. (Better the other way around ?)

Add a new feature:

	param.show changed

will only list parameters with non-default values (in short format)