Refactor measurement containers

[leokoppel]

Further to #41. Code duplication was noted in PR #82.

Reduce code duplication between the similar MeasurementContainer and LandmarkMeasurementContainer (and any other future variations).

This is done through static polymorphism, using CRTP (the technique used extensively by Eigen). References:

• http://eli.thegreenplace.net/2011/05/17/the-curiously-recurring-template-pattern-in-c
• https://stackoverflow.com/questions/4173254/what-is-the-curiously-recurring-template-pattern-crtp
• https://stackoverflow.com/questions/6006614/c-static-polymorphism-crtp-and-using-typedefs-from-derived-classes

Pre-Merge Checklist:

• [x] Code is documented in doxygen format
• [x] Code has automated tests
• [x] Zero compiler warnings
• [x] Formatted with clang-format

Testing done: existing unit tests. There is no behaviour change, so unit tests are untouched.