Fix name lookup for the interpolateParam function

[kingscallop]

Fixes a few errors like the ones below.

On clang:

In file included from spark/src/Core/SPK_Factory.cpp:21:
In file included from spark/include/SPARK.h:33:
spark/include/Extensions/Interpolators/SPK_GraphInterpolator.h:599:3: error: explicit qualification required to use member 'interpolateParam' from dependent base class
	            interpolateParam(result,entry.y0,entry.y1,ratio);
	            ^

On gcc:

spark/include/Extensions/Interpolators/SPK_GraphInterpolator.h:599:33: error: ‘interpolateParam’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
 599 |                 interpolateParam(result,entry.y0,entry.y1,ratio);
     |                 ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~