fix: take absstep into account for FiniteDiff

[gdalle]

Fixes #811

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.46%. Comparing base (d8905f5) to head (3b9665d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ferentiationInterface/test/Back/FiniteDiff/test.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #812      +/-   ##
==========================================
- Coverage   97.93%   97.46%   -0.47%     
==========================================
  Files         128      127       -1     
  Lines        7693     7694       +1     
==========================================
- Hits         7534     7499      -35     
- Misses        159      195      +36     

Flag	Coverage Δ
DI	98.16% <86.66%> (-0.65%)	⬇️
DIT	95.61% <ø> (ø)

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