add more description to ergodic trim
Merge branch 'doc' of github.com:mpharrigan/msmbuilder

Conflicts:
	src/python/MSMLib.py
Check for invalid lagtimes
Make sure inputted lag times are positive
Merge branch 'master' of github.com:SimTk/msmbuilder
Merge pull request #361 from mpharrigan/lagtime_check

Lagtime check