[IMPL] Equivalent stress amplitude functions

[Vybornak2]

Implement equivalent stress amplitude functions for

• [IMPL] HCF uniaxial SWT criterion #40
• [IMPL] Implement stress-life prediction function based on the Goodman mean-stress correction #44
• [IMPL] Implement stress-life prediction function based on the Gerber mean-stress correction #45
• [IMPL] Implement stress-life prediction function based on the Morrow mean-stress correction #46
• [IMPL] Implement stress-life prediction function based on the ASME mean-stress correction #50

In addition in collaboration with @JanHPapuga update docstrings of the function to include applicability, meaning.