Add `dim` argument and convert it to axis?

[rcaneill]

    * [ ] Another nice feature would be to add a `dim` argument for functions needing the `axis` argument, and then transform it to the proper axis, e.g. use `gsw.Nsquared(ds.SA, ds.CT, ds.p, lat=None, dim='depth')`

Originally posted by @rcaneill in #1 (comment)