Allow missing location, elevation and barometer height metadata in station metadata

[david-i-berry]

For mobile platforms there is a requirement to set the location to missing in the input CSV files and through the webform as there is no single fixed value. However, the lat/lon is mandatory, forcing users to enter an incorrect value. These may then be used if the lat/lon are missing in the input BUFR file and using the passthrough / bufr2bufr plugin, leading to incorrectly encoded data. The data also appear at an incorrect location in the webapp (e.g. see #629 (comment))

Similarly, not all platforms have a barometer and so it should be possible to set this value to missing as well.