Mean fatigue offset

[cmichelenstrofer]

Add mean fatigue correction to PostProcessing.jl.

Concepts: Miner's rule, Goodman's correction, Rainflow-counting

Background:

This task would assist in documentation relating to the OWENS.jl post processing sections.

Currently, post processing is all based on classical laminate theory, which is all set up in the pre-processing using precomp. Basically, we take the composite layup with all it's complexity of layer orientation, number of stacks, and offsets, and condense that into 6x6 beam sectional properties. Those are mapped to the mesh, and are used in the various simulations. The beam strains and curvatures are then mapped back to each composite layer and effective strains in that frame of reference are calculated, as are the corresponding stresses. Those stresses are then fed into a rainflow counting algorithm that bins up the number of cycles for each stress bin. Then, we run that through a material stress-cycles to failure curve and Miner's rule to calculate the fatigue damage.

Currently at the last step, the fatigue assumes 0-mean load. The mean correction should be relatively simple, resulting in a few added lines of code, and the mean data should be readily accessible at the fatigue calculation step, but would require understanding of the surrounding processes, which should help with documentation.

To complete this task:

Add the mean offset corrections in a branch in OWENS.jl
Develop a hard coded verification case that isolates the fatigue calculation function, shows proper calculation, and add it to the test suite.
Update/add fatigue calculation to an existing time domain test suite simulation.
Open a pull request against Dev and verify that all updated tests pass