C# Plugin API : GH_ParamManager public instead of protected

Hello,

it would be a very small change with pretty much no impact, but for now GH_InputParamManager and GH_OutputParamManager classes are protected. (inner clases to GH_Component).

It would be nice if they could be made public, since it would then be much easier to build small helpers to create/manage parameters.

Thank you
Julien