ENH: Check for metadata compliance with SPDF metadata standards

[jklenzing]

Related to #6.

A method that checks a pysat data file for SPDF / SPASE metadata compatibility would be useful for data providers, particularly for small missions such as rockets, cubesats, balloons.

[JonathonMSmith]

Here's the ISTP guide from SPDF: https://spdf.gsfc.nasa.gov/istp_guide/istp_guide.html

[jklenzing]

Check meta keys and values for

• non-standard ascii characters
• characters that cannot be used as python variable names (eg, '-')
• non-duplication of values (see ops_reach)