esfit update

[cetait]

Update to esfit function and GUI.

To do:

• [ ] decide on how to deal with supporting different Matlab versions (related to issue #289)
• [ ] implement global fitting (issue #222)
• [ ] investigate and fix Levenberg/Marquardt algorithm (seems to signal convergence after only a couple of steps with clearly bad fits in some cases)
• [ ] decide/implement resizing of GUI window (to fix issues with automatic resizing if esfit GUI used on computer accessed remotely)
• [ ] extensively test GUI for different Matlab versions and screen resolutions

[cetait]

Global fitting and GUI updates implemented in separate PRs.