Added items for new Data Structure:
1. Data Class - defines all regular data expressions
2. MdfData Class - defines data expressions for MDF data
3. NetcdfData Class - defines data expressions for netcdf data
4. XmlData Class - defines data expressions for xml data
5. AsciiData Class - defines data expressions for ascii data

The data class wraps all information about the a site and data from the
site into one class.  Note that all of these classes are for point data.

A future update will be made for grid data classes.