Add "completely symmetric" fourth-order tensors to notation.ExplicitConverter

Exchange of completely symmetric tensors, such as fiber orientation tensors of fourth order in a
minimal, i.e., nearly redundancy-free notation is a common task for engineers working with short fiber reinforced composites.

Support of
```
tensors =

con = mechkit.notation.ExplicitConverter()
con.convert(
inp=tensors, source="tensor", target="exchange", quantity="fiber_orientation"
)
```

with "exchange", e.g., being
```
[
N_1111, N_2222, N_3333,
N_2233, N_1133, N_1122,
N_1123, N_2223, N_3323,
N_1113, N_2213, N_3313,
N_1112, N_2212, N_3312,
]
```
See also equations (38) to (40) in
[Variety of fiber orientation tensors, Bauer and Böhlke 2021](https://journals.sagepub.com/doi/full/10.1177/10812865211057602)

It is clear, that, e.g., N_3333 could be omitted, as only 14 components are independent, but
not omitting it, will enable usage for, e.g., diffusion-weighted magnet resonance tomography tensors of fourth order,
which have no normalized trace.