Add clFFT_GetSize() for getting the estimated size of a plan

This MR adds clFFT_GetSize(), which returns the estimated size of a plan similar to cufftGetSize(). This is useful for comparing the measured memory usage with a memory model, e.g. GCT, ComputeFstat. Different to cufftGetSize() we need also the batchSize, as different to cuFFT for ECLFFT this is not set while the plan generation and is changeable in the execute step. But in our lalsuite GPU functions, this is not changeable after plan generation.

@bema would assign this to you, but I havn't the rights to do this

\cc @ReinhardPrix @hbeggenstein

include/clFFT.h

@@ -194,6 +194,8 @@ cl_int clFFT_1DTwistPlannar(clFFT_Plan Plan, cl_command_queue queue, cl_mem arra
 
 void clFFT_DumpPlan( clFFT_Plan plan, FILE *file);
 
+cl_int clFFT_GetSize(clFFT_Plan Plan, size_t* workSize, cl_uint batchSize);
+
 #ifdef __cplusplus
 }
 #endif