dayofyr and dayofwk after cftime.datetime.replace

[spencerkclark]

#66 enabled working dayofyr and dayofwk attributes for all cftime dates constructed manually, which is great!

I've noticed that if one constructs datetime objects using replace, that the dayofyr and dayofwk attributes are not automatically updated:

In [21]: date = cftime.DatetimeNoLeap(1, 2, 1)

In [22]: date.dayofyr
Out[22]: 32

In [23]: date.replace(year=2, month=5).dayofyr
Out[23]: 32

A workaround is to specify dayofwk=-1 within replace:

In [24]: date.replace(year=2, month=5, dayofwk=-1).dayofyr
Out[24]: 121

Is this intentional? Should we not pass down the old dayofyr and dayofwk attributes to the new date object in replace? I'm happy to provide a PR to change this behavior, if desired.

[jswhit]

I incorporated your workaround into the replace method. If you can come up with something more elegant, feel free to create an alternative pull request.