Consistency w/ 0s in mechanics

Previously, there were inconsistencies in mechanics.essential, with some
methods using S(0) to form empty Vectors and others using an empty Vector
(Vector([])). This has now been corrected to use empty vectors only. The same
is true for the Dyadic class.
Merge pull request #1 from gilbertgede/fixfunc

Consistency w/ 0s in mechanics