suppress unused-parameter compiler warnings

[meyerj]

...in specialized method implementation ParamDescription<std::string>::clamp().

This is required if user code is compiled with -Wunused-parameter, -Wunused or -Wall options. An alternative solution would be to omit the parameter names from the signature.

[mikaelarguedas]

I like the casting approach because it keeps the signature "meaningful" and let users know the semantic of the parameters (even if not used).

Thanks!