Introduce a dynamic state vector

[n-claes]

PR description

This PR introduces a dynamic state vector instead of a length-8 hardcoded vector with perturbed variables.

• separate module mod_get_indices added with index retrieval functions
• boundary conditions: indices are no longer hardcoded but retrieved based on the state vector variable.
• refactored matrix_gridpts to dim_matrix

[codecov]

Codecov Report

Merging #93 (1d88f94) into develop (6506f79) will increase coverage by 0.15%.
The diff coverage is 85.54%.

@@             Coverage Diff             @@
##           develop      #93      +/-   ##
===========================================
+ Coverage    88.17%   88.33%   +0.15%     
===========================================
  Files          102      103       +1     
  Lines         6082     6111      +29     
===========================================
+ Hits          5363     5398      +35     
+ Misses         719      713       -6     

Flag	Coverage Δ
legolas	90.61% <85.54%> (+0.21%)	⬆️
pylbo	83.56% <ø> (ø)

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