add a raws_toRawsDF() function

[jonathancallahan]

The raws_toRawsDF() function will convert a raws object into a tidy dataframe we will calls a rawsDF object. (?Or rawsTidy?)

The rawsDF object will be a tidy dataframe with the following columns:

• nwsID
• wrccID
• siteName
• longitude
• latitude
• datetime
• parameter
• value

This should be enough information to generate some nice ggplot2 maps and time series without overly bloating the size of the tidy dataframe. (A tidy dataframe is orders of magnitude larger than our compact list(meta = ..., data = ...) data model but offers many conveniences such as easy filtering and of course ggplot2.)

This conversion function should do some quick math to estimate what sort of memory footprint the resulting tidy dataframe will have and, if it is going to be larger than some threshold (100MB?), stop() with a warning message to subset the data before converting to a tidy dataframe.

With this rawsDF object available, we can start adding rawsDF_~() functions to work with it.