Multiple StellarPopulation instances

Because parameter state and dirtiness is specific to each instance, but the computed model is held by the module level driver object, having multiple StellarPopulation instances can cause a model to not be recomputed even though it should be (example code below) This might also cause problems with shared memory multiprocessing.

Some documentation advising only a single sps instance per program might be a good idea.

from numpy.testing import assert_allclose
import fsps
sps = fsps.StellarPopulation(zcontinuous=1, vactoair_flag=False)
pop = fsps.StellarPopulation(zcontinuous=1, vactoair_flag=False)
w1, s1 = sps.get_spectrum(tage=10.0)
w, s = pop.get_spectrum(tage=1.0)
w2, s2 = sps.get_spectrum(tage=10.0)
assert_allclose(s, s2) # this should not pass, but does
assert_allclose(s1, s2) # this should not fail, but does

从 StellarPopulation.get_spectrum 和 issue 中描述的模块级 driver 状态开始。使用两个实例复现所提供的序列，然后跟踪参数脏状态和模型重新计算是如何共享的。完成的标准是每个实例都返回与其自身参数一致的结果，包括示例中的两个 assertions。