Enhanced backwards compatibility with previous function

[mellofariam]

• fixed savePositions to work in the new version
• corrected bug when using OpenCL platform (OpenMM platform selection is case sensitive)
• updated saveStructure to work in the new version, using novel nomenclature when saving the files
• corrected saveStructure behavior of ignoring user input filename when saving pdb and gro files
• changed name of printForces to getForces to match new behavior, and added printForces function to output the current forces in the screen
• updated runDefault tests to work in the new version