Fixed the output format of g_principal

Gromacs-4.6 and earlier versions had the output transposed. This
patch fixes it such that paxisN.dat contains the x/y/z components
of the N:th prinicipal axis. The default file output names have
been changed to paxisN.dat to increase the probability that users
who rely on old scripts will need to read the help text and find
the changed format.

Fixes #1441, related to #609.

Change-Id: Ic1ed9370145d3389ae8f43c2a419765dabf3a66f