Potential redundancy in computation of `ln_P_mu` function

[marmarmarmar]

I am curious whether this computation:

https://github.com/Teichlab/SpatialDE/blob/87b908ba30d5981fb825f831cafc30cdeef5060e/Python-module/SpatialDE/aeh.py#L75

might be performed once and taken out of the loop. It does not seem to differ between different loop calls - so one may compute it once and save the computational time.