TST: phase_proc tests skipped if packages missing

test_phase_proc_lp_glpk and test_phase_proc_lp_cvxopt will only be
run when the glpk or cvxopt packages are installed.
BUG: MdvFile cartesian coordinates loose accuracy.

On earlier versions of NumPy (1.5.1) test_mdv.test_cart fails
do to the itermediate arrays having a dtype of float32.

Use extended precision when calculating cartesian coordinate on
MdvFile objects to ensure numerical accuracy of the coordinates