Added a flag for the iTEBD expectation value method.
Removed unused code. slow_string_order can now handle i==j.
Simplified the expectation value scheme so that we can switch between different methods.
Fixed typo
Updated the iTEBD tests to include the new expectation value methods.
Another method of computing expectation values with iTEBD.
MPS_ITEBD_CANONICAL_EXPECTED is now default
Fixed a few typos
Changed the formula for the entropy so that it is always computed with the same vector of coefficients in canonical form.