Improve support for mixed scalartypes

[lkdvos]

This adds and refactors some functionality to improve the coverage of dealing with mixed scalartypes.
In particular, I've added implementations for Base.complex for cases when one wishes to go from <:Real to <:Complex entries.
This is particularly useful for example in the case of a Real groundstate, where time evolution methods would result in complex states, which currently errors.

Fixes #258

[codecov]

Codecov Report

Attention: Patch coverage is 72.00000% with 28 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/operators/mpohamiltonian.jl	61.11%	14 Missing ⚠️
src/states/finitemps.jl	25.00%	6 Missing ⚠️
src/states/windowmps.jl	40.00%	3 Missing ⚠️
src/operators/abstractmpo.jl	66.66%	2 Missing ⚠️
src/operators/lazysum.jl	0.00%	1 Missing ⚠️
src/operators/mpo.jl	97.36%	1 Missing ⚠️
src/states/infinitemps.jl	66.66%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/MPSKit.jl	100.00% <ø> (ø)
src/algorithms/timestep/tdvp.jl	90.14% <100.00%> (+0.14%)	⬆️
src/operators/lazysum.jl	72.00% <0.00%> (-3.00%)	⬇️
src/operators/mpo.jl	90.00% <97.36%> (-0.29%)	⬇️
src/states/infinitemps.jl	64.82% <66.66%> (+0.03%)	⬆️
src/operators/abstractmpo.jl	57.21% <66.66%> (+1.28%)	⬆️
src/states/windowmps.jl	71.60% <40.00%> (-2.08%)	⬇️
src/states/finitemps.jl	77.97% <25.00%> (-1.53%)	⬇️
src/operators/mpohamiltonian.jl	83.60% <61.11%> (-5.36%)	⬇️

... and 6 files with indirect coverage changes