Adapt formatter for ISO durations

[Flix6x]

This PR implements a workaround for an issue I raised with the isodate maintainer(s).

[coveralls]

Pull Request Test Coverage Report for Build 2715104497

• 26 of 27 (96.3%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 67.572%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
flexmeasures/utils/time_utils.py	19	20	95.0%

Totals
Change from base Build 2715071258:	0.06%
Covered Lines:	7201
Relevant Lines:	10083

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💛 - Coveralls

[nhoening]

I guess your point is that timedelta(days=1) has no defined duration, so there can be no "PXD" formatting from that ― the only correct way to treat this is to assume 24h. Is that right?

This seems to work. My only request would be to add a test with assumptions like I tried below, but if we regard this as temporary until isodate fixes it, that is not needed.

In [2]: from flexmeasures.utils.time_utils import duration_isoformat

In [3]: from datetime import timedelta

In [4]: duration_isoformat(timedelta(hours=1))
Out[4]: 'PT1H'

In [5]: duration_isoformat(timedelta(hours=14))
Out[5]: 'PT14H'

In [6]: duration_isoformat(timedelta(hours=24))
Out[6]: 'PT24H'

In [7]: duration_isoformat(timedelta(days=1))
Out[7]: 'PT24H'

In [8]: duration_isoformat(timedelta(days=1, seconds=22))
Out[8]: 'PT24H22S'

In [9]: duration_isoformat(timedelta(days=1, seconds=122))
Out[9]: 'PT24H2M2S'

In [10]: from isodate import duration_isoformat as orig_duration_isoformat

In [11]: orig_duration_isoformat(timedelta(hours=1))
Out[11]: 'PT1H'

In [12]: orig_duration_isoformat(timedelta(hours=14))
Out[12]: 'PT14H'

In [13]: orig_duration_isoformat(timedelta(hours=24))
Out[13]: 'P1D'

In [14]: orig_duration_isoformat(timedelta(days=1))
Out[14]: 'P1D'

In [15]: orig_duration_isoformat(timedelta(days=1, seconds=22))
Out[15]: 'P1DT22S'

In [16]: orig_duration_isoformat(timedelta(days=1, seconds=122))
Out[16]: 'P1DT2M2S'

[Flix6x]

I guess your point is that timedelta(days=1) has no defined duration, so there can be no "PXD" formatting from that ― the only correct way to treat this is to assume 24h. Is that right?

I think that is the right conclusion from the wrong point. datetime.timedelta(days=1) is defined as 24 hours, so it does have a defined absolute duration, whereas "P1D" is defined as a nominal day, so it doesn't have a defined absolute duration.

Good idea to add those tests.