Added x,y,z attributes
Fixed various problems with previous commit
More fixes
FLoat Error
FLoat Error again
Fixed get_mass_properties function
Added Ioxx, Ioyy, Iozz, and made equipment list contain dictionary
Added get_inertia to subsystem, get_total_inertia to spacecraft

Because Ixx, Iyy and Izz can only be calculated once the Cg of the
entire spacecraft has been calculated, the calculations were moved to a
different function.
The new function in spacecraft runs this new function after
get_mass_properties, and determines the total inertia around each axis
for the spacecraft
Include else for Ioxx calc's
Avoid problem with 'shape' = 'Other'
Fixed dictionary calls
now allows equip&subs to both exist
additional fix to last commit
Merge pull request #1 from nswilli2/master

Implementation of mass properties