ENH: add contextmanager to HDFStore (#8791)
Added a context manager to HDFStore

closes #8791

this is fine

pls add a release note in the enhancements section
pls review the docs in io.rst to see if anything needs to be changed
 @jreback I've updated the docs. Not sure if I've got it right.
 pls squash, otherwise looks ok.

`get_store` now looks superfluous. Maybe we should deprecate in future versions?

@jorisvandenbossche 
 @jreback I've added warning that get_store is deprecated
 On the deprecation, I don't know if it is needed to really deprecate it, maybe we could only do "deprecation by documentation" (-> keeping it as an alias (and indicate this in the docstring of `get_store`) but use `HDFStore` everywhere in the documentation)
 @jorisvandenbossche @jreback so I've made those changes. Sorry misunderstood that you guys wanted it deprecated. 
 ok, pls add a release note (put in enhancements section)
 looks ok to me. ping on green.
 @jreback it's green
 @vikram thanks!
