347 missing values

[quantenschaum]

Here is a starting point to fix #347

• check for missing values in NMEA parser and omit then (do not set to zero)
• added parsing of DBK and DBS sentences
• hide course vector if COG is undefined
• hide course vectors in heading mode if zero SOG is detected
• adjusted widgets to display --- as default if values are undefined
• frontend: merge dicts in state model to collect nested additionalKeys

[quantenschaum]

I would like to ask you have a look into this and merge it. I contains some IMHO important fixes.

• The tree of values in the frontend that is kept in sync with the values on the server is not updated correctly for subtrees (like in the sail instrument). This is fixed by cfb8a86
• missing/non-present values should not be set to zero but to nothing. Zero and not value/undefined value are different things.
• missing/non-present values should not be displayed as zero, it should display --- to indicate that there is no value for this property.
• The same holds true for the heading/course vectors on the chart, if there is no data, do not display them.