Fix JuliaBackend calibrate constructor

[nefrathenrici]

• Add calibrate(; ekp = ekp::EnsembleKalmanProcess) functionality
• Improve keyword tests

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (5dd782a) to head (110f3b7).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/backends.jl	0.00%	11 Missing ⚠️
src/ekp_interface.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #116   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          7       7           
  Lines        352     357    +5     
=====================================
- Misses       352     357    +5     

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