Needed to remove my data attributes

[ashjbarnes]

I initialised with a netcdf (Rather than path to file) and it had some basic attributes like a _FillValue of NaN, a long_name and coordinates. Out of the box, Lagrange filter hit the following error:

NetCDF: Attempt to define fill value when data already exists.

Deleting all attributes fixes the issue but I guess this isn't the intended behaviour