Bump file format version

[achilleas-k]

Reminder issue to change the file format version before finalising version 1.5.

Current format version is 1.1.1, which was set after we introduced the DataFrame. Should this stay at 1.1.1 or should we increase it to 1.1.2 for the ColumnDimension #748 and the DataFrame Tag features #717?

[achilleas-k]

For v1.5 the file format version should be 1.2.0 because of the DataFrameDimension. This will block old libraries from opening new files, since they won't be able to read the new dimension type.

With the 1.2.0 file format version we are also introducing a File UUID. A python script is being written to handle upgrading files from old version to the new format. This will also include changing the format of the properties.

See G-Node/nixpy#449.