Get Kelvin notation coefficient of given strain/stress component.
The Kelvin notation [1] is a particular way of building the matricial form
of tensorial quantities.
Parameters:
idx (int (or list[int])) – Index of strain/stress component. Alternatively, a pair of
strain/stress components indexes (associated to a given fourth-order
tensor matricial form element) can also be provided.
comp_order (list) – Strain/Stress components order associated to matricial form.