Scaling and Rotation of PixImages

[luithefirst]

Aardvark.Base leaves the following functions for Scaling and Rotation of PixImages up for the library user to be initialized:
s_scaledFun using SetScaledFun
s_rotatedFun using SetRotatedFun
s_remappedFun using SetRemappedFun

1. We should add meaningful error messages if these functions are not initialized.
2. We should add out-of-the-box implementation without any 3rd party dependencies (would make the error message obsolete).
   I just recently implementation a "Scaled" function in "Tests\Aardvark.Base.Tests\Images\PixImageTests.cs" that could be used a start.

NOTE: A High-Performance implementation can be installed using the Aardvark.VRVis.Images library that uses IPPI Tensor extensions.

[krauthaufen]

Hey, maybe we should just implement these things using our native-tensors.
Linear downsampling is already implemented, but the bicubic/lanczos filters are not...
It's a little odd anyways that PixImage has the API when it doesn't actually work.

[hyazinthh]

Added a default implementation for scaling based on @luithefirst 's implementation. Also these functions throw a proper exception now, if they are invoked and haven't been set.