calibrator: add sim_length param to Calibrator constructor

[AldoGl]

Proposed changes

I add a sim_length parameter to Calibrator constructor.

This is useful if one uses a loss that allows time series of different length to be compared, and wants the simulated time series to be of a specified length.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• I have read the CONTRIBUTING doc
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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works