Robustify `model.steady_state`

[jessegrabowski]

Close #24 by adding a method argument to model.steady_state which can be either root (default) or minimize. Minimize will allow the solver to handle partially solved steady-states.

Add support for symbolic computation of the hessian matrix with method = "minimize".

[codecov[bot]]

Codecov Report

Merging #25 (92a1cf6) into main (c8fcadc) will increase coverage by 0.24%. The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
+ Coverage   55.04%   55.28%   +0.24%     
==========================================
  Files          38       38              
  Lines        4865     4907      +42     
==========================================
+ Hits         2678     2713      +35     
- Misses       2187     2194       +7     

Impacted Files	Coverage Δ
gEconpy/solvers/steady_state.py	60.74% <86.79%> (+4.09%)	:arrow_up:
gEconpy/classes/model.py	55.71% <100.00%> (+0.16%)	:arrow_up:
gEconpy/parser/parse_distributions.py	78.77% <100.00%> (+0.03%)	:arrow_up: