Instantiate runner from config and add CRPS runner

[gmertes]

To support running the CRPS model, which has an extra argument fcstep in the predict_step fn. This argument is only picked up by the new CrpsRunner, and ignored by default.

Any other future extra arguments can be passed in this way, and only used by a specific runner.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.03%. Comparing base (ff2bd59) to head (f57078b).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #106   +/-   ##
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  Coverage    98.03%   98.03%           
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  Files            3        3           
  Lines           51       51           
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  Hits            50       50           
  Misses           1        1           

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