Use caching for isd-history.csv file about gsodr HOT 5 CLOSED

This may not need to be cached. Can use the external data structure in an R package and all the user to update the data in the package manually as desired.

See example of this functionality here: https://github.com/r-net-tools/net.security/blob/master/R/public.R

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Also see: http://stackoverflow.com/questions/14711277/modifying-r-package-data?noredirect=1&lq=1

where it is desired to have latest changes overwritten when new package is installed since it will presumably be the same or newer than the last version updated by the user.

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The devel branch has a test of this functionality now:
5e7562a

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Functionality now included with ab4d245

Still requires the ability to update the internal database and merge with the 90m elevation values

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Fully implemented by #27 and a few prior branch merges mainly including 765741c

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