Suggested XML output enhancements

[merlinND]

Hi @bathal1,

Thanks for the great work on the plugin so far! Here are some small suggestions to improve the output XML when exporting a scene to Mitsuba 2:

• [x] Insert a newline between each section: sampler, integrator, sensor // BSDFS // shapes
• [x] Create a default tag for resx, resy and spp to set them quickly from the command line
• [ ] Create plugin options to pick the relative path where textures, shapes, etc are saved. E.g. some people may want everything in the same directory, others may want to have different directory names, etc.
• [x] Create an option (maybe in the export dialog where we decide where to save the scene) to have it split up into several sub-scene files in a fragments subdirectory (sampler, integrator, sensor // BSDFS // shapes), and then one main file which includes all of them. The idea is that this makes it much easier to replace e.g. just the BSDFs with a manually-modified file rather than having it to carefully insert the changes every time the scene is re-exported.
• [x] For the sensor's to_world transform, consider using a sequence of rotations and translations rather than a single transformation matrix which is harder to modify afterwards.