Implement bases whose elements have finite trace #11
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When dealing with different subspaces, one might want to generate an operator basis that completely separates into elements that live exclusively on the one and elements that live exclusively on the other subspace. This is not possible when forcing all operator bases to have an identity element (and thus the remaining elements traceless). The core feature affected by this change is the way a complete basis is generated from partial orthogonal elements. A switch now controls whether a traceless or non-traceless is generated. Additionally, the checks at the end of _full_from_partial() have been removed. The n elements from which the basis is constructed now are the first n of the final basis. Including non-traceless bases also shone light on a bug in calculate_error_transfer_matrix for a single qubit and the Pauli basis, which did not include contributions from identity elements in case of a cross-correlated spectrum. This induces non-unitality.
This is a major time sink in the calculation of the control matrix using calculate_control_matrix_from_scratch(). Computing real and imaginary part separately and writing to a preallocated array should make the whole calculation faster by a factor of two.
In this case, additional terms need to be considered.
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Having an operator basis whose only traceless element is proportional to the identity is not strictly necessary (it only simplifies a few calculations). However, giving up this restriction allows for separating different subspaces using the bases.