pynml -validate *nml doesn't validate all *nml files at once

[pgleeson]

jnml -validate *nml will validate all nml files in the present dir (and provide a brief summary), but `pynml -validate nml` will run jnml N times, once for each of the N *nml files.

[sanjayankur31]

Note that we can now validate models against the schema without needing jnml (using the validate() methods that we've added to each component type python class). We need to migrate the extra tests over from jnml. This is WIP here: https://github.com/NeuralEnsemble/libNeuroML/pull/156

[pgleeson]

Closing as original issue fixed