Support newer GCC versions and minor cleanup

[victorreijgwart]

Some minor changes to make sure voxgraph compiles cleanly with newer GCC versions, such as on Ubuntu 20.04.

Compiled and tested on Ubuntu 18.04 and 20.04.

Changes

• Stricter compile flags.
• Make sure all class members are initialized in the right order (in each constructor's member initializer list).
• Add default cases to switch statements to avoid warnings in newer GCC versions.
• Remove misleading (ignored) const qualifiers for functions that return values by copy.
• Updates to the .clang-format rules to automatically order #includes.

[alexmillane]

@victorreijgwart Should we merge this? Currently, we're not building on 20.04, and this solves matters :)

See: ethz-asl/voxblox#370 and ethz-asl/cblox#43