0.2.55 for NeuroML v2.1

This is primarily a bugfix release to updates libNeuroML for NeuroML version 2.1.

As always, the simplest way to install libNeuroML is using pip:

pip install --upgrade libneuroml

More information on libNeuroML can be found in the documentation. More information on NeuroML can be found in the NeuroML docs.