make_unique method

[Tokazama]

One of the assumptions of an AbstractAxis is that all of the keys are unique. In order for this to be possible when categorical keys (strings, symbols, etc) are used there should be a way to conveniently handle names that are that are the same. For example, DataFames.jl has a make_unique option to create unique names where they're are multiple instances of the same name when concatenated/joining stuff.

Either there should be a way to automate creation of unique keys, opt into creating unique keys, and/or error when there aren't unique keys.