Issue #797

Forms part of the fix for #797. The main changes are:
* New FilterType mappings for 'Short', 'Decimal', and 'CurvePropertyType'
* Rearranged FilterType filterTypeFromString() logic and tested more thoroughly
* Made the 'type' attribute of Filter required, and removed safe-navigation where it was previously required
* Updated the logic for creating Filters during a WFS scan. Tidier and now respects required 'type' attribute
* Updated and added logging in a few areas