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Note that they all operate on 6x1 matrices. I also added a method to the `Body` object so that it can calculate its spatial inertia at a given point. Also added to body are some empty attributes that will be made use of by the joints that I will be sending a PR for soon.\r\n\r\nI do not know that all of the spatial vector functions will be needed and I will weed out the ones that are not necessary.\r\n\r\nI have some confusion about how he defines his 3x3 rotation matrices. In all of his 3x3 rotation matrices (functions rx, ry, and rz) the negative is not on the same term I have seen in other literature. A quick link to the wikipedia page for [rotation matrices](https://en.wikipedia.org/wiki/Rotation_matrix) shows this discrepancy. @moorepants do you know why this might be? (Featherstone defines these matrices on in table 2.2 in his book on page 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