WIP: Astnodes for interpolation

This PR needs maybe still needs some clean-up. However, it would be good to recieve already some feed-back.

What works:

• Using CUDA textures
• Using HW accelerated interpolation for float32 textures
• Implement linear interpolations either via software (CPU, GPU), texture accesses without HW-interpolation but HW boundary handling
• Adding transformed coordinate systems to fields

What does not work:

• HW boundary handling for CUDA textures for the boundary handling modes mirror and wrap (apparently they have been removed from CUDA's API but are still present in pycuda. Now there's only

cudaBoundaryModeZero = 0
    Zero boundary mode
cudaBoundaryModeClamp = 1
    Clamp boundary mode
cudaBoundaryModeTrap = 2
    Trap boundary mode

Wtf is trap boundary mode? Nothing is documented so we can only experiment.

What kind of works:

• B-Spline interpolation on GPU using this repo as a submodule (http://www.dannyruijters.nl/cubicinterpolation/), to lazy for tests. Don't know how to prove correctness
• Textures for dtypes with itemsize > 4. PyCUDA has helper header (https://github.com/inducer/pycuda/blob/master/pycuda/cuda/pycuda-helpers.hpp) that loads doubles by two int fetches. However, this hack seems to be only working if we add a 0.5 offset and make all functions in this header accept float.

pystencils/astnodes.py

@@ -517,7 +517,7 @@ class SympyAssignment(Node):
 
     @property
     def undefined_symbols(self):
-        result = self.rhs.atoms(sp.Symbol)
+        result = {s for s in self.rhs.free_symbols if not isinstance(s, sp.Indexed)}
         # Add loop counters if there a field accesses
         loop_counters = set()
         for symbol in result: