General improvements

* cleaner interface with gdxdump with Format=csv
** pros: no more unix commands  required
** cons: no more loading of marginals or bounds for variables and
equations allowed, just levels
* Panel5D is now available as a class under gdxpy.pd
* more compact printing of symbols when loaded
* gdxsymb.populate() renamed to gdxsymb.get_values()
* gdxsymb() now support a string as gdxsource