Improve simulation output

[timmens]

In this PR, we change the simulation output from a list of dicts containing the simulated choices (on dict for each period) , to a data frame with multiindex (periods, initial_states_id) and columns corresponding to choices, states and targets (auxiliary variables that might be of interest).

---

After this simulation call

    res = simulate_model(
        params,
        vf_arr_list=vf_arr_list,
        initial_states={
            "wealth": jnp.array([1.0, 20, 40, 70]),
        },
        targets=["utility"],
    )

the output will look like this:

[codecov]

Codecov Report

Merging #36 (ff3ac76) into main (7962873) will increase coverage by 0.06%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head ff3ac76 differs from pull request most recent head a1a43e2. Consider uploading reports for the commit a1a43e2 to get more accurate results

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   98.45%   98.52%   +0.06%     
==========================================
  Files          38       38              
  Lines        1947     1969      +22     
==========================================
+ Hits         1917     1940      +23     
+ Misses         30       29       -1     

Files	Coverage Δ
src/lcm/simulate.py	98.48% <100.00%> (+0.28%)	⬆️
tests/test_simulate.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes