Add MP2 Triples and instantiate 5 and 6D Tensors

[jonasleitner]

For ISR(3), the 2-particle ground-to-excited-state TDM, ADC(4), etc. the second-order MP triples t-amplitudes are needed.

In this PR I enabled 5 and 6 dimensional libadcc.Tensor and instantiated the corresponding tensor operations.
I added (anti-) symmetrisation for index triples and added a make_symmetry_triples factory which will probably be required for a future ADC(4) implementation.
Note that 5 dimensional tensors are not fully supported yet, because I did not add make_sym_bispace_aux<5>.

I also implemented the second-order MP triples as minimal use case to test the implementation.
For this purpose I regenerated the adcc and adcman MP reference data to include the tt2 amplitudes.

[jonasleitner]

As far as I see, the test coverage decreases mostly, because the newly added tensor operations for 5- and 6-dimensional tensors are not used currently... because there is nothing implemented that uses the triples 😅
Meaning the coverage should automatically increase again once we have equations that use them?
So I guess it should be fine for now?