Change the implementation of Transition getComponentVolume to include the volumes...

Change the implementation of Transition getComponentVolume to include the volumes of the shoulders as well as the main body.  This means that Transition no longer needs to override getComponentMass since the default implementation of density * volume now works correctly.

Added helper function to RocketComponent to compute the volume of a ring component by computing the "mass" with density of 1.0.

git-svn-id: https://openrocket.svn.sourceforge.net/svnroot/openrocket/trunk@599 180e2498-e6e9-4542-8430-84ac67f01cd8