Improve ResultsHandler

[mlincett]

Following up on #178 I would like to improve ResultsHandler.

As a first step, instead of using class ad-hoc named attributes to store sensitivity, discovery potentials and other outputs I would like to wrap all this in a single object.

Then I would like to add the calculation of the 3 sigma discovery potential (or at least make the significance threshold configurable).

Questions:

• should the result object have a flat structure (for example fields like sensitivity-value, sensitivity-error, discovery-potential-5sigma-value, etc.) or a nested structure (['sensitivity']['value'], etc.)? While the nested structure is more intuitive, it is also more complicate to implement and may not really make the object more usable;
• should we standardize the structure using native python dataclasses or be bold and open the door to the use of pydantic in flarestack (after @robertdstein suggestion)?

I will start by converting

flarestack/flarestack/core/results.py

Lines 104 to 114 in d93f511

	self.sensitivity = np.nan
	self.sensitivity_err = np.nan
	self.bkg_median = np.nan
	self.frac_over = np.nan
	self.disc_potential = np.nan
	self.disc_err = np.nan
	self.disc_potential_25 = np.nan
	self.disc_ts_threshold = np.nan
	self.extrapolated_sens = False
	self.extrapolated_disc = False
	self.flux_to_ns = np.nan

to a dictionary but I would appreciate some feedback before moving onto more radical improvements.

[JannisNe]

I would really love to use a pydantic model. That way, it would also be really easy to export all the results as a json. I think it should not be that complicated to implement.

[JannisNe]

Re the nested-vs-flat question: I think a nested structure is not that much more useful or intuitive so I think a flat structure is better in terms of complication to usefulness.