Missing validation for SBGN PD layout rules and recommendations

[Drishti160803]

Problem

The SBGN PD specification describes several layout rules and
recommendations but none of these are currently encoded in the
Schematron validation files (sbgn_pd.sch, sbgn_af.sch, sbgn_er.sch).

Examples of unvalidated layout rules

1. Glyphs assigned to a compartment should be visually contained
   within that compartment bounding box
2. A glyph label should be placed within or immediately adjacent
   to the glyph boundary
3. Two glyphs of the same class should not completely overlap
   each other (zero distance between bounding boxes)

Proposed solution

Add new Schematron assertions in sbgn_pd.sch to validate these
layout rules where they can be expressed using coordinates
available in the SBGN-ML format.

Reference

SBGN PD Level 1 specification - layout recommendations section