A shared file with commonly used functions (like isApprox)

For the ctests we have the shared header file OceanTestCommon.h to avoid redefining functions such as isApprox().
For the omega source code itself, we do not have this and different PRs end up rewriting it (e.g. #466).
It may be useful to clean up the code but having such functions in a shared header file in /src/.

A similar question came up in the review of #438 (comment). I already started working on a common MathUtils header.