Add support for Distributed.jl workers

[nefrathenrici]

This PR adds basic support for calibrations parallelized with Distributed.jl's remote workers. This mostly copies SlurmManager functionality from ClusterManagers.jl with some simplifications. ClusterManagers has been unmaintained for a while now and this PR allows us to control and test the features we need for parallelizing calibration.

Core changes:

• set_up_forward_model and run_forward_model are now combined into one function, forward_model
• SlurmManager: subtype of ClusterManager
• worker_calibrate and run_iteration: actual calibration functions
• cleaned up methods for initialize, calibrate, added ekp_constructor

[codecov]

Codecov Report

Attention: Patch coverage is 11.93182% with 155 lines in your changes missing coverage. Please review.

Project coverage is 17.19%. Comparing base (60be971) to head (0b0ef01).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/workers.jl	0.00%	110 Missing ⚠️
src/backends.jl	9.67%	28 Missing ⚠️
src/slurm.jl	0.00%	14 Missing ⚠️
src/ekp_interface.jl	88.88%	2 Missing ⚠️
src/ClimaCalibrate.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            main     #120       +/-   ##
==========================================
+ Coverage   0.00%   17.19%   +17.19%     
==========================================
  Files          7        9        +2     
  Lines        370      506      +136     
==========================================
+ Hits           0       87       +87     
- Misses       370      419       +49     

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