Fix rotate

[drobnyjt]

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Fixes #284

Description

This PR does two things:

1. replaces computed linear algebra with a pre-computed rotation matrix for given n
2. removes panic from rotate_given_surface_normal

Tests

Tested with the following script:

from libRustBCA import *
import numpy as np
delta = 1e-12
num_test = 1000000
mag = np.sqrt((1 - delta)**2 + delta**2)

nx = np.array([(1 - delta)/mag]*num_test)
ny = np.array([delta/mag]*num_test)
nz = np.array([0.0]*num_test)

ux = np.array([-delta]*num_test)
uy = np.random.uniform(-1, 1, num_test)
uz = np.random.uniform(-1, 1, num_test)
umag = np.sqrt(ux*ux + uy*uy + uz*uz)
ux /= umag
uy /= umag
uz /= umag

actually_incident = nx*ux + ny*uy + nz*uz < 0

ux2, uy2, uz2 = rotate_given_surface_normal_vec_py(nx[actually_incident], ny[actually_incident], nz[actually_incident], ux[actually_incident], uy[actually_incident], uz[actually_incident])
ux1, uy1, uz1 = rotate_back_vec_py(nx[actually_incident], ny[actually_incident], nz[actually_incident], ux2, uy2, uz2)

print(np.sum(ux1), np.sum(ux))
print(np.sum(uy1), np.sum(uy))
print(np.sum(uz1), np.sum(uz))