metric_components: signature is ambigious

[fjebaker]

Currently, there are some implemenations that are

metric_components(m::AbstractMetric, x::SVector{4})

and others that are

metric_components(m::AbstractMetric, xt::SVector{2}) # only r and theta components
metric_components(m::AbstractMetric, xt::NTuple{2}) # same as above

The return type is also all over the place.

This is needlessly confusing. Suggest we differentiate metric_components and metric_elements, the prior is used as the implementation, and metric_elements is used as an accessor method?

This also seems complex though...