Lots of cleanup of itensor.h and iqtensor.h. Removed some unused methods. Added isZero methods for checking whether tensors are exactly zero, with a "Fast" option.
Faster implementation of ITensor::takeImagPart that doesn't allocate if imag part is zero. Need to test further that zero scale_ handled properly throughout rest of library, though.
New implementation of MPSt::position which works with only one site tensor at a time to be more efficient.