pygrams requires date of (timeseries_)date_to, date_from in "yyyy/mm/dd" format but this breaks file writing on windows.

[l-bailey-fnc]

Describe the bug
Pygrams requires the data_to, date_from, timeseries_date_to, and timeseries_date_from command line arguments to be passed provided in YYYY/MM/DD format, but this breas the output file creation (on windows at least).

Change to use YYYY-MM-DD format. (Note, this is the superior date format!).

To Reproduce
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4. See error

Expected behavior
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
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