add observing method field

[MathewBiddle]

Within the US MBON community, there is a growing need to group observations into categories based on observing method. This can help researchers identify collaborators and identifying where specific methods are being used. I'm not sure if 'observing method' is the right term, but something to indicate how the observation was obtained. For example, using eDNA, passive/active acoustics, animal tracking, imagery (video/still).

It would be useful to provide a metadata field that had a list of these methods for which you could filter monitoring activities.

EML has a methods section but I can't find a controlled vocabulary to represent this attribute.

[MathewBiddle]

cc @laurabrenskelle

[MathewBiddle]

Some vocabs:

• NASA GCMD - https://www.earthdata.nasa.gov/learn/find-data/idn/gcmd-keywords
• NERC Vocabulary Server - https://vocab.nerc.ac.uk/ (we'd have to do some sleuthing to find the specific vocab in there)