Brace initialization during recycling

[molpopgen]

Recycling code assumes that all objects have explicitly defined constructors:

• gametes
• mutations

If #100 is enacted, then not all valid types will necessarily have constructors defined. Thus, we should use brace initialization here and update the relevant policy documentation regarding how parameter forwarding works.

[molpopgen]

It seems unrealistic to define any useful mutation type that doesn't have a constructor. Closing for now.