Read and write RTB blocks

[jamesmkrieger]

It would be nice to have a file where we can manually supply blocks like in http://prody.csb.pitt.edu/tutorials/membrane_anm/imanm.html#assigning-blocks and be able to read them into ProDy.

It would also be really nice to be able to write this information to file to look at our block boundaries like we can with secondary structures.