DB: standardize keywords

During fixing of sdc-website#49, It became apparent, that keywords in the observation table need to be consistent across different instruments. At least for those keywords where generic searches are performed, we need to decide on a method.

Proposed:

Keyword	Use	Data Type
DATE-BEG	Beginning of observation	Datetime
DATE-End	End of observation	Datetime
THETA	Heliographic Angle	Float
MU	Cosine of heliographic Angle	Float
WAVELENGTH	Wavelength in NM or Angstrom	Float

Implementing this requires:

• interaction with all importers
  • LARS
  • GRIS
  • BBI
  • ChroTel
• Changing or updating names in existing entries
  • LARS
  • GRIS
• Updating website to use only the specified Keywords