use logging

[orbeckst]

For debug and info output we do not want to use print but use the standard logging facility.

[orbeckst]

See https://github.com/alchemistry/alchemlyb/pull/32#discussion_r148339627 for more discussion.

[orbeckst]

We now also use logging for MBAR (see PR #172 ).

[orbeckst]

There are currently no print() function calls in alchemlyb itself.

To close this issue, we only need to add to the docs

• [x] user: how to enable logging
• [x] API doc: use logger with name "alchemlyb.MODULE.CLASS" name space.

We can then keep adding logging where necessary.

[orbeckst]

With PR #303 we now have docs for logging.