Interpolate or insert static values for station coordinates when no GPS is available

[BaptisteVandecrux]

Conversation started here:
GEUS-Glaciology-and-Climate/PROMICE-AWS-data-issues#152
GEUS-Glaciology-and-Climate/PROMICE-AWS-data-issues#148

This implies being able, for the user to separate true observations from interpolated values.

[BaptisteVandecrux]

After we get smoothed and extrapolated coordinates at the level 3 , they could be used as a fallback for the get_bufr.py script when now coordinates can be derived from the transmission file only:

pypromice/src/pypromice/postprocess/get_bufr.py

Lines 280 to 284 in 3958fb1

	logger.warning("Insufficient position data")
	# Don't use any position attributes from latest_data
	station_position["lon"] = None
	station_position["lat"] = None
	station_position["alt"] = None

[BaptisteVandecrux]

fixed in #294