implement layer attribute statistics

Notes:

- only vector layers are operated on (other types are skipped)
- attributes must be a valid XSD numeric type (string, geometry and other types yield 'NA' results)
- default or non-existent derivations are cast as 'NA' results)
- for now, the work interacts with GeoServer's WPS, but not with OWSLib (due to GeoServer response output issues).  Once the GeoServer issue is resolved, I'll port to use OWSLib proper
- skips field names like 'id', 'identifier'

Considerations:

- should we show other values for other data type output (on which numerical aggregations cannot be performed)?
- what other cases should we test for which should tells us to skip an attribute, even if it is numeric?