Version 2.1.1

This version is backwards compatible back to 2.0.0.

All updates to the spec between release 2.1.0 and 2.1.1 can be seen here:
v2.1.0...v2.1.1

There are three minor documentation clarifications, as described in the changelog:

1. clarified how to indicate a dimensionless quantity within an array of units for an array-valued parameter with non-uniform units; see Issue #85
2. clarified the use of scalar and array values for labels and units that describe an array-valued parameter; see Issue #91
3. clarified that null is not allowed as a value within a centers or ranges array in a bins description; see Issue #86