SYNOP_parser and ogimet improvements

[bczernecki]

The task is to create a SYNOP parser in R which may be useful for downloading bigger chunks of data from meteorological stations based on the OGIMET web service.

Requested functionalities:

• parse any SYNOP message
• modify ogimet-related functions to use SYNOP endpoints as default, and parsing HTML content as optional

The logic can be based on Python equivalent: https://github.com/antarctica/pymetdecoder

Other improvements:

• Re-try for unsuccesful downloads in the HTML-based ogimet parser