Add a strftime function to cdtime object

[doutriaux1]

For compatibilty with python's dateutils see: https://docs.python.org/2/library/datetime.html#strftime-and-strptime-behavior

[github-actions]

Marking issue as stale, since there has been no activity in 30 days.

Unless the issue is updated or the 'stale' tag is removed, this issue will be closed in 7 days.

[durack1]

@jasonb5 I think this should be considered as an enhancement for cdtime

[github-actions]

Marking issue as stale, since there has been no activity in 30 days.

Unless the issue is updated or the 'stale' tag is removed, this issue will be closed in 7 days.