Allow user to set a block length for the Circular block bootstrap

[TatianaBurek]

Describe the New Feature

Add a new field in UI and XML to set a block length for the Circular block bootstrap and pass to to agg_stat.py
The request came from the discussion dtcenter/METplus#1659 (reply in thread)

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Issues should represent approximately 1 to 3 days of work.

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Define Related Issue(s)

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• METplus, MET, METdatadb, METviewer, METexpress, METcalcpy, METplotpy

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