Fisheries-HST FloatingPointError: invalid value encountered in true_divide

[phargogh]

This sometimes causes our tests to fail (https://ci.appveyor.com/project/davemfish/invest/builds/28437818/job/y1psai9ejk8h2htg)

It's clear what the problem is, but unclear what the correct scientific solution is.

fisheries_hst.py line 144 divides by array n_a, which is populated by a user-input matrix of 1s and 0s (see lines 109, 110 for n_a, and trace it back to the user input in fisheries_hst_io.py)

fisheries_hst.pyline 110 is especially suspicious: n_a[n_a == 0] = 0

Originally reported by @davemfish at https://bitbucket.org/natcap/invest/issues/3925/fisheries-hst-floatingpointerror-invalid