Investigate alternative partitioning in tridiagonal solver

[msimberg]

https://github.com/eth-cscs/DLA-Future/blob/6ac429e39214190ea9661bbc0877990023092160/src/eigensolver/tridiag_solver/kernels.cu#L638-L643 could potentially be done using a copy and then in-place partition instead. Investigate if it's faster than the current method.

This is low priority unless we find evidence that this kernel is a bottleneck.