Improve ensemble generation of ssesolve and change parameters on stochastic processes

[albertomercurio]

Checklist

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Description

This PR improves the generation of the ensemble problem for ssesolve. Moreover, the stochastic processes were still using the old structure of parameters using NamedTuple. I changed it to be totally generic, with the NullParameters default value.

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.06%. Comparing base (0ce0b45) to head (c56efc1).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/time_evolution/time_evolution.jl	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #403      +/-   ##
==========================================
+ Coverage   93.65%   94.06%   +0.40%     
==========================================
  Files          45       45              
  Lines        2899     2862      -37     
==========================================
- Hits         2715     2692      -23     
+ Misses        184      170      -14     

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