add unit recommendation from iris validator

[crotwell]

fix #73

[pevans-gfz]

75: stronly -> strongly

85: Don't start a sentence with abbreviations "E.g", "I.e" etc. So "...division is specified with '/'. For example ..."

87: More controversially, since we are being SI, meter->metre, at least in the documentation.

[crotwell]

Fixed the first two.

87: More controversially, since we are being SI, meter->metre, at least in the documentation.

Two thoughts on this. First "metre" is SI, but "meter" is very common in existing stationxml, so I am not sure if using metre in docs adds to confusion or not.

Second, I would prefer to sidestep the language issues completely by preferring SI symbols to names. So perhaps add:

SI symbols are preferred over names to avoid language differences, for example `m/s` is preferred over "meter/second" or "metre/second"

But given how much existing stationxml exists with unit names, this may also be controversial.

[pevans-gfz]

Your wording there (m/s is preferred over "meter/second") makes great sense to me, and is in line with your earlier #51.

[jschaeff]

I'm in favor of m/s wich is clearly consistent with the international system of units https://en.wikipedia.org/wiki/International_System_of_Units

And it should appear in the unit recommandations.

As it is "only" a recommandation, it's OK to push best practices, it won't force anyone to change the existing.