Fix small reference flow values instead of strict zero

[geofjamg]

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Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

When computing the post contingency reference flow of a line which is in series with another line which is itself the contingency, we would expected to post contingency active power flow to be zero. Instead we have a very small value.

What is the new behavior (if this is a feature change)?

Does this PR introduce a breaking change or deprecate an API?

• The Breaking Change or Deprecated label has been added
• The migration guide has been updated in the github wiki (What changes might users need to make in their application due to this PR?)

Other information:

[geofjamg]

How to solve this? The way post contingency reference flows are calculated in a DC sensi analysis make it quite logic to not have strict zero values but a very small values (like 1e-13) because it is calculated from the pre-contingency reference flow substracted from contingency lost flow * sensi. So we cannot really end up to a perfect zero.
On the other end, for a user point of view (which is not supposed to be aware to how it is calculated), it does not really make sens to not have zero.
What about, as a post processing, filtering very small values of post contingency reference flows and replace it by zero?

[sonarcloud]

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