Remove unnecessary directory level in vtu output

[stephankramer]

In our vtu outputs when writing a pvd, it's written to outputdir/name/name.pvd where name=Elevation2D, Velocity2D, etc. Originally this made sense as the series .(p)vtu files were then be grouped together in outputdir/name/. However these days firedrake also adds an additional level, and so so now outputdir/name/ only contains outputdir/name.pvd and an additional name/ subdirectory , so we get outputdir/name/name/name_0.vtu which seems excessive.

I propose to simply remove the additional level that Thetis adds: i.e. we get outputdir/name.pvd and outputdir/name/name_0.vtu, etc. This has the additional benefit that all .pvd files from a single run end up at the same level in outputdir/ which makes it easier to navigate in paraview.