Implement ISO calendar instant parsing

[bonjourmauko]

Part of #966
Depends on #1229

Here is what I did:

from openfisca_core import periods
periods.instant("2024-W01")

Here is what I expected to happen:

To work.

Here is what actually happened:

InstantError: '2024-W01' is not a valid instant. Instants are described using the 'YYYY-MM-DD' format, for instance '2015-06-15'.

Here is data (or links to it) that can help you reproduce this issue:

#966