The tool fails when input time series include np.nan for distribution-based methods

[btschwertfeger]

Describe the bug
The tool is not able to process time series with only nan values. (quantile_mapping, quantile_delta_mapping, detrended_quantile_mapping)

To Reproduce
Import CMethods and run an adjustment using datasets containing only nan values.

The error is like:

Traceback (most recent call last):
  File "biasadjust.py", line 152, in <module>
    main()
  File "/Users/bts/opt/miniconda3/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/Users/bts/opt/miniconda3/lib/python3.8/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/Users/bts/opt/miniconda3/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/bts/opt/miniconda3/lib/python3.8/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "biasadjust.py", line 130, in main
    result = cm.adjust_3d(
  File "/Users/udo/bts/miniconda3/lib/python3.8/site-packages/cmethods/__init__.py", line 247, in adjust_3d
    result[lat, lon] = func(
  File "/Users/bts/opt/miniconda3/lib/python3.8/site-packages/cmethods/__init__.py", line 1167, in quantile_delta_mapping
    xbins = np.arange(global_min, global_max + wide, wide)
ValueError: arange: cannot compute length

Expected behavior
If one of the time series of the control period (obs, simh) is nan, there can't be made an adjustment and the functions should return the raw simp/scenario time series without further adjustment.