Seting default color for NaN in `addScalarQuantity`

[berendbaas]

While debugging scalar quantities, I find myself often overlooking NaN values, because as it stands, they (seem to) get mapped to zero. It'd be nice if we could specify the color that NaN values should map to, so they're easier to distinguish. perhaps as an extra optional<glm::vec3> argument (if empty, default to current behaviour) argument would help.

[nmwsharp]

I like this idea, I've wanted it myself!

I tried implementing it in https://github.com/nmwsharp/polyscope/pull/312 . Unfortunately, I'm having getting any sane/consistent behavior from isinf() and isnan() in glsl shaders, so it doesn't really work.

[nmwsharp]

I merged #313 which adds a popup warning for nan/inf values in input arrays.

(we will see if it is too annoying for users to be enabled by default :) )