Isochrone File Name Backward Compatibility; Changed Global Random Seed to Random Number Generator of Each Class

[wei-lingfeng]

• Added backward compatibility of isochrone file names.
• Changed the global random seed to a random generator within each class, but still retaining the reproducibility.
• Added support for specifying the random number generator in generate_cluster, calc_multi, and the testing function
• Added documentation on the behavior of the random number generator.
• Updated the comparison star systems and companions for future comparisons accordingly.
• Moved IMF testing functions under spisea/tests

Random number generator is now an attribute of the IMF and Cluster classes:

cluster1 = ResolvedCluster(..., seed=42)
cluster2 = ResolvedCluster(..., seed=42)

cluster1 == cluster2.

For IMF:

imf = IMF(..., seed=42)
result1 = imf.generate_cluster()
result2 = imf.generate_cluster()
imf = IMF(..., seed=42)
result3 = imf.generate_cluster()
result4 = imf.generate_cluster()

result1 == result3, result2 == result4
but result1 ≠ result2, result3 ≠ result4.
This is the same behavior as

rng = np.random.default_rng(seed=42)
result1 = rng.random(1)
result2 = rng.random(1)
rng = np.random.default_rng(seed=42)
result3 = rng.random(1)
result4 = rng.random(1)

If identical output is desired over each run, the random state can be reset before running the function, e.g.,

imf.rng = np.random.default_rng(seed=42)
result1 = imf.generate_cluster()
imf.rng = np.random.default_rng(seed=42)
result2 = imf.generate_cluster()

In this case, result1==result2.