Add Inputs.xml definitions to schema and examples.

Also, add simpleType for resistance to schema, and improve the
relevant LEMS simulation example.

Note that Inputs.xml is still not valid LEMS, as spikeGeneratorPoisson
uses a DerivedParameter to calculate the average ISI only once (rather
than at each timestep).
Merge pull request #15 from NeuroML/inputs

Add Inputs.xml definitions to schema and examples.