Small QoL updates - Githubissues

PR with few quality of life updates

Changelog v3.0.1 (#203)

Solver now automatically generates VTK files for Models and Gradients at the end of each iteration
- New function solver.specfem.make_output_vtk_files that generates .vtk files for all files in the output/ directory
- solver.finalize() runs make_output_vtk_files at the end of each iteration
- new solver parameter export_vtk controls whether the above option is run, default to True, only available for SPECFEM3D/3D_GLOBE
Improve organization for files exported to path_output
- Optimization stats file and figures saved the workdir/output/optimize whereas before they were saved directly to output/
- Changed pyaflowa export directory to workdir/output/preprocess (before it was workdir/output/pyaflowa)
Quality of Life Updates
- path_data directory is deleted by seisflows clean iff empty, since it is created by solver.setup()
- Logging updates: better visual demarcation of workflow tasks, headers relate to workflow function run
- seisflows debug has a cleaner log message
- shuffled workflow finalization procedures so that base class (forward) contains standard finalization procedures
Bugfix
- Uncommented gll model check which had been commented for devel purposes