Fix float display while writing xml document.

 * The older method caused a loss of float precision when writing urdf
   file.
Do not strip trailing zeros in float display.

 * Stripping trailing zeros causes the removal of exponents if value
   is expressed in scientific notation (e.g. 1.2e-10 -> 1.2e-1).
Fix inertia matrix constructor.

  * ixy was set to iyy by mistake.
  * Reported by Joseph Salini.