whitespace cleanup, mention pip install --user

              Configurational log-weight calculation requires pymbar.

              Here are some options to install it:

              conda install -c omnia pymbar

              pip install pymbar

              pip install --user pymbar

              sudo pip install pymbar

              To install the dev. version directly from github, use:

              pip install pip install git+https://github.com/choderalab/pymbar.git