Default but not mandatory values in spec

[anowacki]

For attributes which in the spec have a default value but are not mandatory, decide whether or not to instantiate the structs with the default value or use missing. The former would convey what the spec means those values to be; the latter would convey what is actually in the file.

Latitude is an example, where it may optionally have a datum attribute whose value is by default "WGS84". That is, if no value for datum is present, it is assumed that the latitude is on the WGS84 ellipsoid. At present a file may contain

<Latitude>0</Latitude>

and this gets parsed into

Latitude(value=0, plus_error=missing, minus_error=missing, measurement_method=missing, unit="DEGREES", datum="WGS84")

Note that <Latitude datum="WGS84">0</Latitude> would parse to the same thing, so the question is whether this is right.