initial updates to GeoModelXML docs

Documentation for GeoModelXML.

Some sections are not yet filled in, but they are put in place to show what should probably be there. Others are fairly complete.

Quite a bit is based on slides written by @ncalace for the tutorial days.

EDIT: To-do, where to describe "envelope" mechanism...

FYI @sroe @xilin @rbianchi @boudreau @tsulaia

documentation/docs/components/geomodelxml/components/gmx2geo.md

+# Gmx2Geo
+
+`Gmx2Geo` is the main top-level class of `GeoModelXML`, and as the name suggests, it is the interface through which `GeoModel` objects are created from the `GeoModelXML` processing of the input XML files. It takes as inputs:
+
+- The XML files to be processed
+- A "world" volume into which the geometry tree to be built will be inserted
+- a `gmxInterface` to be used for communication with downstream code (or simply debugging)
+- flags to steer behvaiour (e.g. whether to read from a file or a string from a database)
+- a list of additional `processors` to be run on top of the "standard list" in `GmxUtil` (the main "driver" class)
+
+The primary uses of `Gmx2Geo` are therefore in intergrating `GeoModelXML` in a more general software framework as the method of building `GeoModel` detector description, or in writing plugins which can be used by the other `GeoModel` tools such as `gmcat`, `gmex`, etc.