Adding more metadata to fits files and EISMap

[MJWeberg]

Copied additional header keywords to the EISCube.meta['mod_index']. This will automatically propagate the metadata out to any output "fits" file or EISMap

Short list of keywords added:

• "tr_mode" (Hinode tracking mode)
• "SAA" (IN / OUT South Atlantic Anomaly)
• "HLZ" (IN / OUT High-Latitude Zone)
• "exptime" and "cadence" (avg values for all raster steps)
• "tl_id", "jop_id", "study_id", "rast_id" (EIS specific id numbers)

Also set the uncertainty types of EISCube and EISMap objects to StdDevUncertainty, since AstroPy still lacks a proper class for measured uncertainties, and added default data masks. (closes #55 and closes #84, respectively)

Furthermore, all new fits files created with export_fits will now include extra binary tables with the exact observation and exposure time for each raster step (i.e. slit position along the x-axis). Like uncertainties, these arrays are only read when the fits file is loaded directly with eispac.EISMap. The arrays can be accessed with EISMap.step_date_obs and EISMap.step_exptime. Loading the files with sunpy.map.Map (of just loading older output fits files) will yield estimated times based on date_obs and date_end.

Lastly, added copies of the date keywords to all new fits files with the "_" characters replaced with "-". This gives full compliance with the FITS-4 standard while maintaining the customary keywords used in solar physics and the Hinode mission. (closes #100)