Transformer without Pysteps param

[alanseed]

I have developed a transformer class to manage the data transformations. This way the data transformation and its metadata are kept so that they are not needed in the xarray attributes. I am passing np.ndarray into the class for transformation. The tests have been included.

[codecov]

Codecov Report

❌ Patch coverage is 22.80702% with 88 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.58%. Comparing base (abd1f52) to head (c1c884c).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
pysteps/utils/transformer.py	22.12%	88 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #503      +/-   ##
==========================================
- Coverage   84.56%   83.58%   -0.99%     
==========================================
  Files         162      164       +2     
  Lines       13471    13769     +298     
==========================================
+ Hits        11392    11509     +117     
- Misses       2079     2260     +181     

Flag	Coverage Δ
unit_tests	83.58% <22.80%> (-0.99%)	⬇️

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