OCL_INSTALL_FULL_PATH changed to OCL_INSTALL_DIR

OCL_INSTALL_FULL_PATH - completely removed.
OCL_INSTALL_DIR - stores a path relative to the main install folder.
Added findDefaultDataPath to also work for JIT files

findDefaultDataPath is a generalization of the existing
findDefaultLibraryDataPath function. It works for finding both JIT files
and share/top data files.
Fixed possible runtime error in detect_gpus

detect_gpus was returning -1 if clGetDeviceIDs was failing for any of
the detected platforms. It should ignore the error and continue with the
next platform.
Added OCL_INSTALL_FULL_PATH

Restored OCL_INSTALL_FULL_PATH.
Used in findDefaultJITDataPath as a fallback path.
OCL kernels reorganization

1. Moved all OpenCL kernels in mdlib/nbnxn_ocl
2. Removed ishift.h and nbnxn_consts.h from the list of OpenCL
dependencies - instead, all the macros needed by the kernels are sent at
runtime to the OpenCL compiler with -D
3. Cleaned-up ocl_compiler
4. ocl_compiler assumes all OpenCL code is stored in one single folder
5. Cleaned-up nbnxn_consts.h
6. Updated cmake files