Enabled filters for layer OWS service

Due to possible values being fetched each time it is neater and easier
in code to build the xml from the filter objects each time rather than
parse the stored XML and insert the possible values into each filter node.
Pulls possible values for string filters and sends them in the response
Move document building into separate class
Refactoring
Use FeatureTypeInfo for database agnostic layer property interrogation

SQL layers did not work correctly because there is no column information
for them.
Use the API correctly for a more robust way of locating a layer data directory
Make filter possible value reads database agnostic by using the API
Merge branch 'master' into ows-filters-service
Use helper method to get the file path
Remove unused methods and classes
Prevent empty value elements from being included in filters
Merge pull request #2 from aodn/ows-filters-service

OWS Layer Filters Service