Bug fix for conflict between dask array and np.where

[lee1043]

When dask import, use dask.array.where instead of np.where. This should resolve #143 and #122. The solution was originally suggested by @lfery and I extended it to consider the has_dask indicator (True when dask imported, and False for the opposite).

[lee1043]

@ajdawson I wonder if this fix could be merged to the main and you could release updated version, e.g., v1.4.1. If so, that would be a huge help for my workflow. Because otherwise, I have to consider hosting the capability of eofs in my package directly instead of leveraging on your package that has been considered as one of major dependencies of my package.