EML

Discussion of stmml has revealed several isses, one of which is the fact that
units, as expressed by stmml, are applicable only to measurable quantities.
Many variables that ecologists put in an eml dataset and might intuitively
appear to have "unit's (geologic age, sex, or species names, for example) do
not have units and thus must be decleared "dimensionless" or "undefined"
because unit is required for all attributes. I dont think its intuitively
apparent to users that these are domains not units and that they should be
described as such.

in the required element <measurmentScale> we class all attributes as nominal,
ordinal, interval or ratio. Srictly speaking only interval scales have units,
the rest are dimensionless. In practice, there is still some value of knowing
the units of the denominator and/or numerator in ratios of two dimensions, so
we probably dont want to throw out the baby with the bath water there.

To help clarify this, we might consider merging units within measurementScale
so that things may be set requred when relevant. an example might be:

<measurementScale>
<interval>
<standardUnit>
metersPerSecond
</standardUnit>
</interval>
</measurementScale>
a variant does away with embedding custom units in additionalMetadata would be:
<measurementScale>
<interval>
<unit library="http://ecoinformatics.org/emlUnitDictionary.xml">
metersPerSecond
</unit>
</interval>
</measurementScale>

this would mean any custom unit definitions would need to be published online.

content model for measurement scale might look like:
element measurementScale(nominal | ordinal | interval | ratio)
element nominal
element ordinal
element interval (unit)
element ratio (i'm not sure what would go here - it seems like we're hacking
unit definitions in emlUnitDictionary for ratios already but maybe that should
be pulled out and we provide a structured ratio definition here that references
two (or more?) true dimensions)

all attributes would still have a domain element - the existing bug on that
still applies