Remove deprecated property ModelCheckpoint.period in favor of ModelCheckpoint.every_n_epochs

[Tshimanga]

What does this PR do?

Removal of ModelCheckpoint.period in favor of ModelCheckpoint.every_n_epochs as scheduled for 1.5

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[carmocca]

Thank you for your effort!

[Tshimanga]

@carmocca let me know if you guys want a @every_n_epochs.setter as well. I didn't see any functionality depending on @period.setter anywhere so I left that one out

[carmocca]

@carmocca let me know if you guys want a @every_n_epochs.setter as well. I didn't see any functionality depending on @period.setter anywhere so I left that one out

I thought about it and I don't think we should add it, as there's some post-processing on it (__init_triggers and __validate_init_configuration) so we don't want users to change the value after __init__