improve activation time constant feature

[AurelienJaquier]

Description

• when the time interval is empty, return a vlaue of 0 ms instead of the default value of 1 from curve_fit
• make time interval and voltage interval start at the min voltage value. Motivation: some traces can have a artefact of rising voltage at the injection time. To avoid taking that into the fit, we should take the min value after the artefact
• some fit fail and return a very big return value. Detect them when they are more than 10 times the duration of the interval and return the interval duration instead. It is not perfect, but will still induce less bias if this value is used later on.

Checklist:

• Unit tests are added to cover the changes (skip if not applicable).
• The changes are mentioned in the documentation (skip if not applicable).
• CHANGELOG file is updated (skip if not applicable).