Fix CMake building and linking of nvblox lib

[helenol]

Still not 100% there but, main changes:

• nvblox now built as external project within CMake
• link to nvblox::nvblox_lib target instead of nvblox_interface
• set up include paths

Still to fix:

• [ ] Figure out what parts of all the stuff are necessary
• [x] Get binaries to compile
• [ ] Move STDGPU includes to a proper place
• [ ] Get install targets correct

[tutunarsl]

Tested on another system, builds

[helenol]

Self-LGTM