Factor out session handling from GUI

[paulmueller]

Session handling should be performed in a dedicated class. The export and import of session data (including Correlation, Trace, etc. and all the parameters) should be available through a single interface that can then be used independent of the GUI (e.g. for more elaborate tests).

• The session class contains a list of its measurements

• Each measurement is what corresponds to a "Page" in the GUI (Maybe subclass Page from such a new Measurement class)

• Affected Sources:

  • https://github.com/FCS-analysis/PyCorrFit/blob/master/pycorrfit/openfile.py
  • https://github.com/FCS-analysis/PyCorrFit/blob/master/pycorrfit/gui/page.py
  • https://github.com/FCS-analysis/PyCorrFit/blob/master/tests/test_session.py