HLLD Refactor

HLLD Refactor

I finished refactoring the HLLD solver to use structs instead of scalar variables this week (PR #237). It uses a few less registers now, is much more readable, and several hundred lines shorter.

Other

• Fixed minor bug in Sod shock tube tests (PR #238)
• Removed using namespace std from Cholla which resolves Issue #233 and lets us enable the google-build-using-namespace clang-tidy check. (PR #245)