MapStore improve Graticule layers and implement printability

[mbarto]

We want to implement a vector layer to show graticules (for at least lat-lon and regular UTM zones coordinate systems), with labels attached to it.

The goal is to show the layer in printed maps, but since we already have a GraticuleLayer we should expand that with:

• support for labels and grids (we have already done something in webmapper, so we could start from that implementation, here: https://github.com/geosolutions-it/webmapper-halliburton/blob/DEV/js/utils/GraticuleWithLabel.js)
• printing support (through a transformer that will produce a regular vector layer payload, as we already do for annotations)

Additional requirements:

• the grid used will depend on the CRS chosen for printing (EPSG:3857, EPSG:4326 or UTM)
• we want the grid size to be configurable, and different at different zoom levels (e.g. degress, minutes or seconds for EPSG:4326; meters or km for UTM)
• we want the labels format to be configurable also, and possibly different for the different sizes

Caveat: in printed maps, labels should be external to the map area. To avoid implementing something on the mapfish engine for this, we could add a (white) frame as a vector layer, with labels on it, giving the illusion of external labels.