feat: updating viewer to use base class

This new structure should make it easy to implement different plotting
packages without having to write much code.
Simply inheret from BaseModelPlotter and reimplement the functions
* _add_surface
* _add_vector_marker
* _add_points

LoopStructural/visualisation/__init__.py

@@ -17,8 +17,12 @@
 except:
     logger.error('Cannot use sphinx scraper, pip install -r docs/requirements.txt')
 try:
-    from .model_visualisation import LavaVuModelViewer
-except:
+    from .lavavu import LavaVuModelViewer
+except ImportError:
     logger.error("Missing lavavu, can't import LavaVuModelViewer")
 
+try:
+    from .vtk_viewer import VtkExporter
+except ImportError:
+    logger.error("No meshio")
 from ._scalar_field import ScalarField