Add require statement for surf.data.generate function

Require the use of the 'lubridate' package for some
date/time calculations.
Init list object before loop structure

Needed to add a constructor to generate an empty list
object. Otherwise, there would be no place for the
dataframes.
Change wind direction missing indicator

Changed from 9999 to 999 for a missing integer value.
Corrected missing indicators for reals

Changed from 9999 to 999.9 for these real numbers.
Replace missing data indicator for RH

Changed from 9999 to 999.9. This is for situations
where there is no RH in the data file at all.
Replace precip code missing value

Should be 999. Changed.
Changed RH data missing indicator

Changed from 999.9 to 999.0.
Create standalone function for NCDC history file

The 'get.ncdc.history' function has been made to
better modularize the codebase.