Update objective printing for IDMRG

[lkdvos]

Here I try to have a middle ground solution for the IDMRG logging procedure, see also #315.

The idea here is that previously we were simply counting the "total energy" of an ever growing system, which isn't necessarily that useful. Unfortunately, actually computing the current energy of the infinite state would be somewhat painful as well since it would require computing environments at every step. Here I simply define the current energy as the difference in energy between the last iteration and the current one, which might be a slightly better measure, although it might not be variational etc.
At some point it might be worth it to think about "normalizing" the DMRG environments to subtract these linearly growing contributions, but I think this is a good temporary middle ground.

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/algorithms/groundstate/idmrg.jl	94.73%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/algorithms/groundstate/idmrg.jl	98.55% <94.73%> (+0.16%)	⬆️

... and 1 file with indirect coverage changes

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