Add only_use_cftime_datetimes flag to num2date

[spencerkclark]

Closes #37

I tried to go for the path of least resistance. I added a flag to DateFromJulianDay that allows one to bypass the logic that uses datetime.datetime objects in certain situations (and exposed that flag in netcdftime.num2date). If only_use_netcdftime_datetimes is True we have the following behavior:

• For calendar='standard', DatetimeGregorian objects are always used
• For calendar='gregorian', DatetimeGregorian objects are always used
• For calendar='proleptic_gregorian', DatetimeProlepticGregorian objects are always used

If only_use_netcdftime_datetimes is False (which is the default), then the original behavior occurs.

Does this seem like something reasonable? I'm happy to discuss things further.

[spencerkclark]

In looking at this logic, I noticed that as written this branch of the if-statement is used for both calendar='proleptic_gregorian' and calendar='gregorian'. Therefore, while the next branch is elif calendar in ('standard', 'gregorian') it will never be used in the case of calendar='gregorian'. Was this intended?

In other words, should this line be if calendar == 'proleptic_gregorian'?

[spencerkclark]

Appveyor seems to be failing in setting up the environment for Python 2.7, so I suspect it is unrelated to the changes I've made here (the same thing seems to be happening in #38).

[spencerkclark]

This is now updated in accordance with the renaming of the package to cftime; the option in num2date is now called only_use_cftime_datetimes.

[jhamman]

@spencerkclark - Thanks, this looks good. I'd like @jswhit to have a quick look before merging but generally, its nice to have the added consistency!