Reduce code duplication sources that can be used by the accumulate source #552
Description
The mars and FDB sources have some duplication in how they generate dates/requests when passed an IntervalsDatesProvider instance (i.e. when used by the accumulate source.
This issue covers consolidating the aforementioned code.
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I would add a comment in the fdb source and in the mars source pointing that some code has been duplicated there.
Originally posted by @floriankrb in #550 (comment)