Add `precipitation_type`

[jdemaeyer]

Blocks: #85

• Determine common precipitation types across all observation types
• Add to MOSMIX parser
• Add to SYNOP parser
• Add to current observations parser
• Add to recent precipitation observations parser
• Add tests
• Write migration (including new enum for the precipitation type)
• Add to database exports
• Add to docs sample response

[jdemaeyer]

Common types are none, rain, sleet, snow.

MOSMIX and Current observations also have codes for fog, hail, and thunderstorm (of which only hail is an actual precipitation type...), but those are missing in the formOfPrecipitation field in SYNOP and Recent observations.

I'm tempted to split this into two fields precipitation_type and significant_weather, where the latter will contain fog, hail, and thunderstorm (and be empty for recent/historical weather) and form the base for the icon field.