Cf-python

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**2018-02-13**

* Fixed bug in `cf.Field.collapse` when doing climatological time
collapse with only one period per year/day

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**2018-02-09**

* Fixed bug in Variable.mask

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**2018-02-09**

* Added override_calendar method to coordinates and domain ancillaries
that changes the calendar of the bounds, too.
* Fixed bug in `cf.Data.where` when the condition is a `cf.Query`
object.
* Fixed bug in `cf.Variable.mask`

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**2018-02-07**

* Allowed `scipy` and `matplotlib` imports to be optional

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**2017-11-28**

* Added ``group_span`` and ``contiguous_group`` options to
`cf.Field.collapse`

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**2017-11-10**

* Disallowed raising offset units to a power (e.g. taking the square
of data in units of K 273.15).
* Removed len() of `cf.Field` (previously always, and misleadingly,
returned 1)
* Fixed setting of cell methods after climatological time collapses
* Added printing of ncvar in `cf.Field.__str__` and `cf.Field.dump`
* Added user stash table option to ``cfa`` script

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