Change the way time is added to a model

[rubencalje]

This pull request changes the way time is added to a model:

time = pd.date_range('2000', '2000-1-4')
model_ds = nlmod.mdims.set_model_ds_time(model_ds, time=time)

<xarray.Dataset>
Dimensions:  (time: 4)
Coordinates:
  * time     (time) datetime64[ns] 2000-01-01 2000-01-02 2000-01-03 2000-01-04
Data variables:
    *empty*
Attributes: (12/13)
    time_units:                DAYS
    start_time:                1990-01-01 00:00:00
    nstp:                      1
    tsmult:                    1.0
    steady_start:              1
    steady_state:              0

When steady_start is True, a steady state stress-period is added before the first value of time with a stress period length of steady_start_perlen. The current ways to specify time (transient_timesteps or perlen) are still supported.

The variable time now always contains the time at the end of each stress period (not sure if this was the case before), so heads and other output can be easily added to the model_ds. To minimise double information, nper and perlen are removed from the attributes of model_ds, and are calculated when needed.

[OnnoEbbens]

Nice improvements!

I did add something else when I went through the changes. I moved the attributes with time information ('time_units', 'start_time', 'nstp', 'tsmult', 'steady_start' and 'steady_state') from the model_ds to the time DataArray. So instead of model_ds.attrs['time_units'] = time_units you get model_ds.time.attrs['time_units'] = time_units.

I tested locally and it worked. So when the test pass this can be merged.

[rubencalje]

Nice improvements!

I did add something else when I went through the changes. I moved the attributes with time information ('time_units', 'start_time', 'nstp', 'tsmult', 'steady_start' and 'steady_state') from the model_ds to the time DataArray. So instead of model_ds.attrs['time_units'] = time_units you get model_ds.time.attrs['time_units'] = time_units.

I tested locally and it worked. So when the test pass this can be merged.

Good change. Maybe we can then also rename 'time_units' to 'units' and 'start_time' to 'start'?

[OnnoEbbens]

Good one, added this in 8e54b1b

[OnnoEbbens]

Actually it was not that easy. The 'units' attribute of the time coordinate seams to be locked and I get this error when I try to write it to a netcdf file: ValueError: failed to prevent overwriting existing key units in attrs on variable 'time'. This is probably an encoding field used by xarray to describe how a variable is serialized. To proceed, remove this key from the variable's attributes manually. It is also mentioned here: pydata/xarray#3739. I don't have time now to dive into this. so I will revert the commit for now and create an issue for this.