When people need to serialize in a format that differentiates string concatenation from addition, they can use the new `concat` function in an expression. But I don't think we need to throw if people use `+` with strings.
This work is based on the ol.expr package by @tschaub.
It adds a few named expression functions to that package:
* INTERSECTS, CONTAINS, DWITHIN, WITHIN (no client-side implementation as yet)
* LIKE (like comparison with wildcard, singleChar and escapeChar)
* IEQ (case-insensitive equality)
* INEQ (case-insensitive non-equality)
It also adds a few extra parameters to the existing EXTENT function to be able
to deserialize and serialize all info (i.e. projection and property name).
Some changes were needed for the GML parser as well:
* createGeometry function needed to be public
* when parsing Box (GML2) and Envelope (GML3) also parse the srsName
* fix up writing for Box and Envelope now that bounds is an array
Also added createDocumentFragment function to the XML parser. Implementation
is similar to OpenLayers 2.
Some addtional notes on the implementation:
* PropertyIsBetween was implemented as an ol.expr.Logical with operator
ol.expr.LogicalOp.AND and two ol.expr.Comparison instances with operator
ol.expr.ComparisonOp.GTE and ol.expr.ComparisonOp.LTE
* In OGC Filter And and Or can contain more than 2 sub filters, so this
is translated into a hierarchy of ol.expr.Logical
