Various (hopefully final) fixes (again)

[lkdvos]

Fixes the periodic_boundary_conditions functions, and small other bugs that came to light while rerunning the examples and the README

[codecov]

Codecov Report

Attention: Patch coverage is 2.11268% with 139 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/algorithms/toolbox.jl	0.00%	93 Missing ⚠️
src/operators/abstractmpo.jl	0.00%	29 Missing ⚠️
src/operators/mpo.jl	0.00%	7 Missing ⚠️
src/operators/mpohamiltonian.jl	0.00%	7 Missing ⚠️
src/algorithms/changebonds/optimalexpand.jl	25.00%	3 Missing ⚠️

Files with missing lines	Coverage Δ
src/algorithms/groundstate/idmrg.jl	68.10% <ø> (-30.21%)	⬇️
src/environments/abstract_envs.jl	70.27% <100.00%> (-21.63%)	⬇️
src/algorithms/changebonds/optimalexpand.jl	34.54% <25.00%> (-65.46%)	⬇️
src/operators/mpo.jl	4.94% <0.00%> (-89.18%)	⬇️
src/operators/mpohamiltonian.jl	49.29% <0.00%> (-41.46%)	⬇️
src/operators/abstractmpo.jl	4.45% <0.00%> (-34.61%)	⬇️
src/algorithms/toolbox.jl	14.49% <0.00%> (-78.85%)	⬇️

... and 56 files with indirect coverage changes