CSV loading, refactoring, gui

[WillRNaylor]

Is your feature request related to a problem? Please describe.
In #110 we fixed a bug where we were trying to load csv not hdf5, but now we can't load hdf5 there. The code snippet is at the start of
class stats_trim_dlg(QMainWindow): from silcamgui.py.
There is however a function in
class PlotView(QtWidgets.QWidget): from interactive_summary.py. The function is old_data_check().

So the problem is simply that in the bugfix (which was essenrtially this):
diff: - self.stats = pd.read_csv(stats_filename)
diff: + self.stats = pd.read_hdf(stats_filename, 'ParticleStats/stats')

did not allow for backward compatibility.

Describe the solution you'd like
The solution is probably a small->large refactor. Where maybe we create a class that inherits from df's and then we make custom loading and saving functions for those dfs. The additional problem is this loading function needs some gui support (in interactive_summary.py we automatically ask the user in a pop-up box if they want to convert an old csv file), so it would have to be broken down somewhat, and really this gui functionality should be in a central place, but guicalcs.py maybe isn't correct.

Describe alternatives you've considered
The shit solution is that you just reproduce the function in both classes. Clearly this code duplication isn't good.