Save statmap significance fits information

[GarethCabournDavies]

The significance fits information gets discarded and there is not way to get it back - this change saves the fit coefficient, its approximate error and the rate of triggers above threshold as attributes in the statmap output files

Standard information about the request

This is an information loss bug fix
This change affects the offline search
This change changes: output which can be used for later review / plotting

This change follows style guidelines (See e.g. PEP8), has been proposed using the contribution guidelines

Motivation

We want to be able to get the FAR calculation extrapolation values for review etc.
We can reverse-engineer fit coefficient and count above threshold using e.g. the FAR / stat values above the threshold, but not the error on fit coefficient

Contents

Add a significance info dictionary as output from the get_far function
Report the values in this dictionary into the attributes of the various statmap job outputs

Testing performed

tested for sngls_statmap outputs, ensuring that these match reverse-engineered outputs

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