Deprecate convert(AnchoredInterval{Ending}, ...) and convert(AnchoredInterval{Beginning}, ...)

[omus]

As discussed in another PR the covert methods using AnchoredInterval{Ending} and Anchored{Beginning} should be dropped as:

1. Ending / Beginning is not a period
2. You can't have an instance of AnchoredInterval{Ending} or AnchoredInterval{Beginning}
3. The invariant convert(T, x)::T should always hold in every convert method

With that said, these convert methods have been very useful for generic testing in test/comparisons.jl so we should find an alternate way of providing this functionality.