Schematron-like checks to add

[prushforth]

• check the min/max types and range of input type=zoom compared to the tcrs' definition
• when extent units=WGS84, should input units=gcrs, or does it make no difference?
• check that in the case of an input returning lat/lon, the min latitude is >= -90 , and the max latitude is <= +90
• check that in the case of an input returning lat/lon, the min longitude is >= -180 and the max longitude is <= +180
• in general check that min < max for input type=location
• for input type=width or height, ensure min and max are different and non-zero