Code cleanup for LLVM compilers

[drsteve]

Main items in clean-up:

• Type casting of structs and function in Lgm_SummersDiffCoeff.c (closes Issues (incompatible arguments) in calls to Lgm_SimpleRiemannSum  #48 and closes Incompatible type in GSL handling in Lgm_SummersDiffCoeff.c #49 )
• Update deprecated HDF5 function H5Dextend, replace with H5Dset_extent (closes H5Dextend (in Lgm_HDF5.h) is deprecated (and deprecated symbols are excluded) #45 )
• Add comment for deprecated H5Fis_hdf5 (replacement function requires 1.12 and I don't want to bump the minimum version past 1.10 yet)
• Add function prototype for Lgm_KdTree_Free (closes Function Lgm_KdTree_Free needs defining in header #46 )
• Excludes GPR.c from the build (see Numerous issues in types, return in GPR.c #47)
• Update README.md with a basic install instruction and clearer redirection to HACKING

[JeffreyBroll]

Looks good to me - I just keep forgetting that _qpInfo is int... README changes also great

[JeffreyBroll]

It built fine with clang and gcc. I don't have tests to run off the top of my head, but there's nothing in Lgm_SummersDiffCoeff that isn't what was implicitly done so that should be fine.

[drsteve]

Thanks! The CI builds with GCC (although that worked before these changes), and I've built with cce-clang. I'll wait for Sean's feedback and then merge.