Changes to _default_method

[mikeingold]

Background

The _default_method utility function currently takes a (::Geometry) or (::Type{<:Geometry}) as argument since it depended on the geometry being integrated. PR #139 potentially makes this obsolete by removing Analytical and making FiniteDifference the universal default.

Ideas

I think the function should be renamed _default_diff_method for clarity.

Should this function continue to take geometry-information as an argument? We're not currently using that information, but it's possible that could change again in the future as more DifferentiationMethods are implemented.

[JoshuaLampert]

I think it doesn't harm if _default_method takes a geometry. It shouldn't decrease performance, but allows for more flexibility in the future. I agree that renaming it to _default_diff_method makes sense.