pt.py: z2pt: changed behaviour for singular matrices: skipping instead of exception

Signed-off-by: LK <LK@uni>
pt.py: changed handling of skipping back out of function z2pt - exception handled within the PT object itself - modified functions edi2pt and z_array2pt

Signed-off-by: LK <LK@uni>
pt.py: changed attributes z,z_object to methods set_z and set_z_object, because it is not possible to obtain a unique full Z from a PhaseTensor

Signed-off-by: LK <LK@uni>
Merge branch 'master' of github.com:geophysics/mtpy