Add optimization flag for release to cmakelists

[TimSiebert1]

This PR consists of

• "O3" flag for 'CMAKE_BUILD_TYPE=Release'
• include the corresponding command to README.md

Was suggested by @juanlucasrey

[juanlucasrey]

Hi Tim, thanks for the PR.

/home/runner/work/ADOL-C/ADOL-C/ADOL-C/boost-test/traceOperatorVector.cpp(2851): error: in "trace_vector/FmaxOperator_FOV_Forward_3": check yd[0][1] == aDerivative has failed [-0 != 1]: absolute value exceeds tolerance [|1| > 1e-08]
/home/runner/work/ADOL-C/ADOL-C/ADOL-C/boost-test/traceOperatorVector.cpp(2987): error: in "trace_vector/FminOperator_FOV_Forward_1": check yd[0][1] == bDerivative has failed [0 != -3.7000000000000002]: absolute value exceeds tolerance [|-3.7| > 1e-08]

Funny you get those errors. -O3 should be safe in the sense that it does not do unsafe optimisations. (for unsafe optimisations usually "-Ofast" is used, although it is not recommended)

These differences are not small, so they could be due to uninitialised variables somewhere. I can check further into it if you don't have time.

[TimSiebert1]

Hi Lucas,

this error is actually not related to the optimization flag. The test failed before too. If I remember correctly, there should be a mistake in this test. I wanted to fix it this week, but if you have time to work on it, we would appreciate it.