Modernizing printing the functions.

This makes the solid, segment, and human printing nicer with precision control
and supression of zeros using NumPy's set_printoptions function.
Fix test_inertia.py:878 test.
Fix test_inertia.test_print_properties test for new print functions.
Fix test_inertia.test_print_solid_properties test for new print functions.
Human.__str__ method prints human inertia properties.
Segment.__str__ method prints segment inertia properties.
Fix kwarg bug in Segment.print_solid_properties.
Merge pull request #81 from chrisdembia/print-format

[WIP] Modernizing printing the functions.