Added support for reading in addition .series files.

Supported extensions added in this commit are:

vtm.series
vtmb.series
vthb.series
vth.series
vtp.series
vtt.series
vtu.series
vti.series
vtk.series
vts.series
vtr.series
pvtp.series
pvtu.series
pvtt.series
pvti.series
pvts.series
pvtr.series
pvtk.series
ply.series
stl.series

A test for the vtp.series extension is included.