Merge pull request #3 from mlewe/three_site_h

Add expect_3s() for three-site operators.
Fix bug in new take_step.
Improve clarity of take_step.
Improve clarity of take_step a bit more.
Detect truncation with example.
Add expect_1s_1s to mps_uniform. Catch another possible exception in getting nrm2.
Make sandwich tdvp work again allowing only 2-site hamiltonians for now.
Update readme.