Fixed number of steps

[xuanxu]

This PR adds a couple of options to have time integration using different time steps for stars with m < 4Msun and m > 4 Msun.

The integration_step: setting will accept two new values: The different time steps can be calculated automatically using the two_steps_t option, or the user can specify a fixed number of steps for each integration interval with the fixed_n_stepsoption, and then set the values using two new settings:

• a fixed number of time steps for massive stars (M* > 4Msun), specified with the integration_steps_stars_bigger_than_4Msun setting

  • a fixed number of time steps for small stars (M* < 4Msun), specified with the integration_steps_stars_smaller_than_4Msun setting

[codecov[bot]]

Codecov Report

Merging #27 into master will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
+ Coverage   99.69%   99.71%   +0.02%     
==========================================
  Files          11       11              
  Lines         648      714      +66     
  Branches       94      102       +8     
==========================================
+ Hits          646      712      +66     
  Partials        2        2              

Impacted Files	Coverage Δ
src/intergalactic/settings.py	100.00% <ø> (ø)
src/intergalactic/model.py	100.00% <100.00%> (ø)

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