Parse timestamp strings in heuristicate-types, issue #14

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Hello,

This is a draft implementation of the behavior proposed in issue #14. Now heuristicate-types replaces timestamp strings with timestamp objects using local-time.

I've only recently started studying lisp-stat and may miss some details. Please let me know if my approach to this task is correct. I'll be glad to hear any feedback, fix the issues and do all the work necessary to merge this feature.

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Alexander Kuzmin

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I have read the CLA Document and I hereby sign the CLA

[snunez1]

I think this is generally good, although I'd rather throw an error if local-time can't parse the string. I.e. we want to catch problems earlier in the pipeline than later.

[tre-flip]

I'd rather throw an error if local-time can't parse the string

Done. I've also made it map to a new vector first to prevent inconsistent modifications to the dataframe if an error is signaled halfway through.