Add better default methods for interface methods

[DanielVandH]

Currently I define default methods for specific types, e.g. getx(::AbstractVector), but perhaps its smarter to just define getx(p) = p[1] as a default and trust that users can redefine if needed, rather than throwing a MethodError. If this is done, I should provide a function that allows a user to test their interface matches the package's assumptions correctly.

Should also check if any methods really need to exist or can be better defined. Really just need to put a lot more thought into this overall for the longevity of the package.