feat(config): Add accumulate_from_start_of_forecast post-processor

[gmertes]

Implements step 1 and 2 of #131

For CLI and OO API users: if post_processors is not defined in the user-config, the current behaviour is to keep accumulating from the beginning of the forecast. A warning is shown to alert users that this will change in the future.

To explicitly turn accumulations from beginning of the forecast on or off, the user can do:

# on
post_processors: 
  - accumulate_from_start_of_forecast

# off
post_processors: []

For the numpy-to-numpy API it is turned off by default.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.07%. Comparing base (90728d5) to head (09b7b13).
Report is 46 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
- Coverage   98.03%   96.07%   -1.97%     
==========================================
  Files           3        3              
  Lines          51       51              
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- Hits           50       49       -1     
- Misses          1        2       +1     

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