Add notes on limitations of OpenSearch Geo & Time

[letmaik]

OpenSearch Geo & Time has a very broad scope and is meant for describing the URL structure of search engines with spatial and/or temporal filtering functions. The terms defined in that spec are restricted to WGS84 coordinates and RFC3339 Gregorian times. In coverage APIs there may be different requirements such as:

• using spatial coordinates in the native CRS of the coverage (or the reprojection CRS as in W*S)
• searching for times beyond the 4-digit year limit (paleoclimate datasets)
• having more than one time dimension (model time and forecast time in weather forecasts)

The OpenSearch Geo & Time terms may be used directly for collection filtering if the above restrictions do not apply. However, for subsetting a coverage, the terms may only be used indirectly as the syntax for (currently) custom terms, since filtering in general is not subsetting and both operations may be used in a single API query, hence the need to differentiate between them. As noted in #4, the distinction between filtering and subsetting is not always clear though (correct me if I'm wrong).

The above notes should be added to the spec to guide implementers.