Replace SIGNATURE with mgard::SIGNATURE

[ben-e-whitney]

This commit replaces the signature macro defined in include/mgard-x/Metadata.hpp with the mgard::SIGNATURE constant defined in include/format.hpp.

@JieyangChen7, please review this commit. In addition to the signature switch, it also removes the Metadata::signature data member (since the only valid value is mgard::SIGNATURE). I can keep that member if you prefer.

@qliu21, please don't merge this pull request yet. I'll rebase on top of fix-installation-paths once you merge #194.

[ben-e-whitney]

@JieyangChen7, just a reminder to take a quick look at this when you have the time.

[JieyangChen7]

@JieyangChen7, just a reminder to take a quick look at this when you have the time.

@ben-e-whitney Sorry about the delay. Just got back from vacation. I will review this by the end of this week.

[ben-e-whitney]

No problem, @JieyangChen7. Hope you had a good vacation.

[JieyangChen7]

@ben-e-whitney I tried to build this PR but it gives me this error with enabling CUDA:

/home/jieyang/dev/MGARD/include/utilities.tpp:248:29: error: ‘__T288’ was not declared in this scope const CartesianProduct<T, N> &iterable,
I think this is the NVCC compiler bug we encountered before. Is there any way we can avoid including "utilities.hpp" when including "format.hpp" ?

[ben-e-whitney]

@JieyangChen7 Please try again with the commit I just pushed (b6f5330). On that commit, on Summit, build_scripts/build_mgard_cuda_summit.sh ran successfully for me.

[ben-e-whitney]

Thanks, @JieyangChen7. @qliu21, this is ready to be merged.