Avoid adding adding exact zeros in TaylorCluster implementation

[leburgel]

The addition of SparseBlockTensorMap slices in the TaylorCluster implementation lead to the explicit instantiation and subsequent purging of explicit zero blocks due to the getindex-setindex! interplay in expressions like slice[linds[s_c], 1, 1, :] += slice[c_lin, 1, 1, :]. This PR circumvents this by explicitly restricting to the nonzero keys of the right hand side. It's not the cleanest solution, but it gives a big performance increase for Hamiltonians with a lot of channels.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/algorithms/timestep/timeevmpo.jl	84.66% <100.00%> (+0.68%)	⬆️

[lkdvos]

Test failures seem unrelated, merging this.