API v3

[matthiasgomolka]

As mentioned in #51, the new API V3 is now mandatory. This PR accomodates for the respecting changes. This is a rather rough PR since not all features are fully tested and there might be other smaller issues.

[codecov]

Codecov Report

Attention: Patch coverage is 25.20000% with 187 lines in your changes are missing coverage. Please review.

Project coverage is 20.44%. Comparing base (39c0de7) to head (8b3c49e).

Files	Patch %	Lines
R/sfa_load_statement.R	0.00%	53 Missing ⚠️
R/sfa_load_shareprices.R	0.00%	48 Missing ⚠️
R/utils.R	6.97%	40 Missing ⚠️
R/sfa_load__shares_outstanding.R	0.00%	35 Missing ⚠️
R/check_inputs.R	10.00%	9 Missing ⚠️
R/zzz.R	33.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #53       +/-   ##
===========================================
- Coverage   92.71%   20.44%   -72.28%     
===========================================
  Files          13       10        -3     
  Lines         508      318      -190     
===========================================
- Hits          471       65      -406     
- Misses         37      253      +216     

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