Better handling for atomman and ovito optional deps

[thomas-rocke]

• Create new [dislocation] qualifier for installing optional deps for matscipy (i.e. pip install matscipy[dislocation] would install atomman and ovito)
• Better fallbacks for missing imports? (e.g. CubicCrystalDislocation.view_cyl falling back to mode="cle" if ovito not available, change the default method of CubicCrystalDislocation to `"adsl"?)