Fixes isodate converter

[actlikewill]

• This allows the converter reverse function to work with datetime objects.

closes #1669

[longhotsummer]

Minor: we're now repeating the same code in two places. A better approach would be to get the value into the expected format (datetime) if it's not there already:

if isinstance(value, str):
    value = datetime.strptime(value, "%Y-%m-%d")
elif not isinstance(value, (datetime, date)):
    raise ValueError(value)
return value.strftime("%Y-%m-%d")

[actlikewill]

@longhotsummer agreed, converting the values to datetime and raising errors for all others is a better approach, updated