Approximate / force upgrade modes for Mitsuba 1 files

[tszirr]

Adds two command line flags (-D__load_permissive=1 and -D__upgrade_approximate=1) to the XML loader, which allow to ignore 'unreferenced X' errors and approximately replace currently unsupported features, respectively.

This PR still subsumes #90, since however I split the changes, they will create merge conflicts, if merged simultaneously. Therefore, to merge both, the simplest option is to only merge this one and close the other one.

The approximate load (-D__upgrade_approximate=1) mode can do the following:

• Transform phong BSDFs and phong NDFs to beckmann w/ approx eq. alpha
• Replace binary mixture BSDFs
• Create proxies for coatings
• Bypass unsupported bumpmap / normalmap nestings
• Unroll shapegroups / instances (About shape groups/assemblies #22)
• Try to convert spectrums w/o wavelength specifications to RGB

Additionally, standard upgrades are extended by:

• Converting hex color codes
• Bump -> bumpmap (which approx load then bypasses until a plugin is available)
• diffuse::diffuseReflectance -> diffuse:reflectance

All these features were required by at least one actual Mitsuba 1 scene file on the internet.

For the sunsky required by many scenes, see the old updated PR (#85).

[wjakob]

Sorry, I'm not a big fan of this change. Mitsuba 2 aims for compatibility with Mitsuba 1, but it is still under heavy development, so compatibility is subject to a "best-effort with what is already implemented" policy. I wouldn't want to add flags like__upgrade_approximate just to get a Mitsuba 1 scene to parse. Of course it's fine to have these types of modifications, but I don't think they should go into the master branch.

[wjakob]

As always, I am happy to discuss further, but I'll close this PR for now.