Make <gateHHrates ...> perferred format aot <gate type="gateHHrates"...>

[pgleeson]

Both of these are valid a/c to the NeuroML 2 schema & are understood by Lems. However, the automatically generated APIs (libNeuroML & org.neuroml.model) have more difficulty with the 2nd format.

• Schema should be updated to ensure type is used correctly whenever 2nd form is used
• Hint given in schema that 2nd is preferred form
• Conversion script for ChannelML -> NML2 should be updated to generate 2nd format
• Examples on OSB (e.g. here) should be updated to the 1st format
• A later release (e.g. beta5) should make 2nd form compulsory

[pgleeson]

One problem encountered when making this conversion was that currently in the Schema a channel has to have gateHHrates, gateHHtauInf, etc. in a certain order, see https://github.com/NeuroML/NeuroML2/blob/development/Schemas/NeuroML2/NeuroML_v2beta4.xsd#L455.

However the channel's gates don't always respect this order, so some NML2 channels converted to this new format were invalid. Therefore they have been reverted to the "old" form, which means a list of <gate> elements is present and each of these can have any of the allowed type attributes.

The ideal solution to this would be to allow any order for gateHHrates, gateHHtauInf, etc. inside ionChannel. This has resulted in problems in the past with the autogenerated APIs. This has been solved by @borismarin in the new work on an updated LEMS schema, and should eventually be incorporated in the NeuroML2 schema.