0.3.5-beta.1

• Improves handling of groups with undefined classes, i.e, those not defined by the NeXus standard.
• Adds an environment variable, NX_DEFINITIONS, which can be used to define the path to the NeXus definitions directory.
• Updates the NXroot and NXobject classes to latest NIAC proposal.
• Fixes an issue with parsing groups with NXroot as parent.