pTrace does vectors

The pTrace function now accepts state vectors as well as density
matrices.  When vectors are given, they're converted to density
matrices, then traced out. The result is still a density matrix.
Resolves #24.
v1.2.1

See release notes. pTrace works on vectors.
Updated examples

The example code has all been updated to take advantage of features from
v1.2.0.  Things are much much more concise with the new simulator
options.