Reweigh SPE charge by angular response function

[kmtsui]

This should be merged after PR #9

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Used with the WCSim type digitizer, the PR adds a reweight function to the SPE charge response in the form of "angular response". The logic is:

1. for each PMT, import a "angular response" coefficient from a txt file as the program starts
2. in the SPE charge function, for each true photon hit, calculate the cosine of photon incident angle relative to the PMT normal (costh), retrieve the "angular response" coefficient for that PMT (ar), then multiply the SPE charge response by a weight of w=1+(costh-0.5)*2*ar

By setting the coefficient txt file, we can implement systematic change to the coefficients based on the PMT positions, orientations, etc.