ENH: Support more of the pandas str accessors

[toddrjen]

Currently pandas supports a lot of str accessors that xarray doesn't. Many of them have useful functionality. I think it would be good if the xarray had more of these str accessors. I would be willing to begin working on this.

There seem to be three categories. One that has a one-to-one mapping between input and output (so one input element becomes one output element), and one that has a one-to-many mapping (one input element becomes multiple output elements), and one that has a many-to-one mapping (multiple input elements become one output element). Exactly how the one-to-many mapping should be handling, if at all, is an open issue. Some of the many-to-one mappings (specifically join) would need a dimension argument.

One-to-one

• casefold(self)
• normalize(self, form)

One-to-many

• extract(self, pat[, flags, expand])
• extractall(self, pat[, flags])
• findall(self, pat[, flags])
• get_dummies(self[, sep])
• partition(self[, sep, expand])
• rpartition(self[, sep, expand])
• rsplit(self[, pat, n, expand])
• split(self[, pat, n, expand])

Many-to-one

• cat(self[, others, sep, na_rep, join])
• join(self, sep)

[dcherian]

This sounds great. Looking forward to those PRa!