Release 1.2.1: Non-Extensible Data Sets

**Proposed release text:**

This is a patch level release of libSplash, [version 1.2.1](https://github.com/ComputationalRadiationPhysics/libSplash/blob/master/CHANGELOG.md)

We changed the default formatting for data sets to be non-extensible in all modes but *append*.
This updates the internal file format to version **2.1** which is as always backwards compatible with the major version (all `2.x` formatted files).