1.2.1: Non-Extensible Data Sets

This is a patch level release of libSplash, version 1.2.1

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 file's major version (all 2.x formatted files).

We are now using Zenodo to add a DOI to our tags/releases.