Include plot of Principal Strains

[talledodiego]

With this PR I included a few features:

• add the possibility to plot principal strains as contour
• add the possibility to add title to the plots
• add the possibility to specify the output resolution and the size of the figure
• add the len function to the Fields object returning the number of frames saved (useful for instance to save all pictures in a loop after doing the dic analysis).

I worked over the dev branch because I wanted to use the implemented csv exporting and save_path which were not available in master branch.

[PolymerGuy]

Thanks for the nice new features! I'm a bit busy at the moment but will review the code as soon as possible.

[talledodiego]

Thanks for the nice new features! I'm a bit busy at the moment but will review the code as soon as possible.

I perfectly understand. Thanks for your effort. Let me know if I need to change something.

Diego