renamed library and header to 'eclfft' to avoid conflicts with clFFT

For the OpenCL version of the Resampling FStat we want to link this OpenCL FFT library together with the clFFT (https://github.com/clMathLibraries/clFFT , see https://gitlab.aei.uni-hannover.de/einsteinathome/lalsuite/issues/5). This PR therefore renames the library and headers to 'eclfft' (E@H's OpenCL FFT) to avoid naming conflcts with the clFFT. Note: this will require to change all code that uses this library, but that should only concern the BRP search code.