`basePointCurrent` is missing in the XSD

[sanjayankur31]

The xml defs state that an ExplicitInput is anything that extends basePointCurrent, but the XSD does not include this at all, which makes it confusing. A PulseGenerator extends basePointCurrent in the XML, but Standalone in the XSD/python libNeuroML.

So:

• should basePointCurrent be listed in the xsd somewhere?
• if not, what can we do to improve the documentation so that it links better with the python/xsd?