Ttclust

Two new arguments added:
- `axis` to force "Frame" label in the barplot axis (use `-axis frame`)
- `limitmat` to decrease the trajectory size limit to generate the distance matrix. It is now `100000000` but if the distance matrix is too long to generate, you can just decrease this limit with `-limitmat 10000`

The version 4.7.3 contains several bugfixes.

2 new functionalities:
- multiple trajectories reading (trajectories will be concatenated).
To read multiple trajectories, just add them after the "-f" argument (like "-f traj1.xtc traj2.xtc" or "-f traj*.xtc)
- striding (read every *th frames) with the "-s" argument ("-s 10" to read every 10th frames)

bug fixe in saving the representative frame

The GUI is now removed. If you want to use the simple GUI, please install it with `pip install ttclustGUI`.
wxPython may cause issues on Mac & linux without X server.

Two bugs fixed
- empty linear output for linear barplot
- MAC ONLY : custom "which" command to replace shutil.which (not
implemented before Python 3.3).