Space transformations refactor

[Micki-D]

This PR incorporates the changes from PR #409, and also includes structural changes of the type structure for mcmc samplers.

E.g.:
"-Iterator" -> "-State"

"Sampling" and "Tuning" are structs that hold the config data for a MCMC sampling and tuning Process, and for the actual execution of the sampling, "MCMCState", and "TunerState" objects are created that hold the concrete data for the process.

[codecov]

Codecov Report

Attention: Patch coverage is 79.75460% with 132 lines in your changes missing coverage. Please review.

Project coverage is 48.39%. Comparing base (3767f31) to head (eb950ea).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/samplers/mcmc/mcmc_algorithm.jl	44.00%	28 Missing ⚠️
...amplers/mcmc/mcmc_tuning/mcmc_adaptive_mh_tuner.jl	74.64%	18 Missing ⚠️
src/samplers/mcmc/mcmc_tuning/mcmc_noop_tuner.jl	0.00%	17 Missing ⚠️
src/samplers/mcmc/mcmc_tuning/mcmc_ram_tuner.jl	72.34%	13 Missing ⚠️
src/samplers/mcmc/mcmc_utils.jl	48.00%	13 Missing ⚠️
src/samplers/mcmc/mcmc_state.jl	93.49%	11 Missing ⚠️
src/samplers/mcmc/chain_pool_init.jl	86.27%	7 Missing ⚠️
ext/ahmc_impl/ahmc_tuner_impl.jl	88.37%	5 Missing ⚠️
src/samplers/mcmc/mcmc_sample.jl	81.81%	4 Missing ⚠️
src/measures/bat_pushfwd_measure.jl	50.00%	3 Missing ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #449      +/-   ##
==========================================
+ Coverage   47.36%   48.39%   +1.03%     
==========================================
  Files         112      118       +6     
  Lines        5422     5558     +136     
==========================================
+ Hits         2568     2690     +122     
- Misses       2854     2868      +14     

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[oschulz]

Let's use samplingalg instead of sampling for argument and variable names, it's clearer.