doable methods for readSBML and writeSBML

[exaexa]

Target functionality

This kind of code may look much better than what we have now:

someCompartments = readSBML("myModel.xml") do m
  m.compartments
end

writeSBML("myModel.xml") do
   compartments=...
   species=...
   SBMLModel(comparmtents, species, ......)
end

cc @giordano