Fix a bug in SphereVelocity::transformFromPS method
Handle one time level field specially

When a field has only one time level, the time level index should be
omitted from API.
Merge constructor arguments into create method in Field classes

In order to avoid tedious pointer dereference when accessing element in a
Field pointer, I merge the constructor arguments into 'create' method so
that the field object could not be delared as a pointer in a class.