Issues 82 and 83: Use term generics and add generic defaults with stop message

[athowes]

Description

This PR closes #82 and #83.

I've changed:

• file name from methods.R to generics.R
• separate file for defaults.R and separate family tag
• add defaults for almost all generics with error message about not having a method for that class
  • this raised point of uncertainty about how to handle epidist_prepare and epidist generic defaults (these do not provide unimplemented class warnings currently): creating issue for this Add error on no method for class in cases where we use default #92

Checklist

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[seabbs]

LGTM. The issue with prepare stems from us abusing the default method as a class constructor.