Synapses changed to use Children for plasticityMechanism & blockMechanism, not Child

This means 0 or more plasticityMechanisms/blockMechanisms can be present
aot Child, where supposedly exactly one should be present (though jLEMS
& PyLEMS didn't compleain if none was present; jLEMSDev did...)
Updated examples to work better with new version
Updated LEMS_NML2_Ex15_CaDynamics.xml to catch initialisation case where
alpha & beta could be 0 => infinite t
Merge branch 'development' of github.com:NeuroML/NeuroML2 into development