Support pickles from previous versions of param

[ceball]

I.e. be able to open pickles of parameterized objects from previous versions of param.

[jbednar]

I think this is very important, and doesn't seem like a big job.

[ceball]

In general, yes, it's easy to implement. But it requires a bit of time to import and/or set up tests (i.e. I think doing this task in any kind of worthwhile way requires some tests to be set up first).