Allow jLEMS to include files specified for inclusion in a NeuroML file

[pgleeson]

Have jlems parse & handle <include href=...> statements like these:
https://github.com/OpenSourceBrain/GranuleCell/blob/13156c204dc0835d873e1e27881b94d4cf32b81d/neuroConstruct/generatedNeuroML2/GranuleCell.net.nml

instead of requiring repetition of all the includes in the LEMS file (using <Include file="...") like here:

https://github.com/OpenSourceBrain/GranuleCell/blob/13156c204dc0835d873e1e27881b94d4cf32b81d/neuroConstruct/generatedNeuroML2/LEMS_GranuleCell.xml

[pgleeson]

Has ben fixed a long time, e.g. https://github.com/OpenSourceBrain/PospischilEtAl2008/blob/master/NeuroML2/LEMS_FiveCells.xml