Small update to the Fourier series potential fitting algorithm.
Small update to the Fourier series potential fitting algorithm.

The least-squares fitting algorithm now adds an extra row which forces the
first derivative of V(phi) to be zero at phi = 0.

pierrelb/RMG-Py,comocheng/RMG-Py,nyee/RMG-Py,nickvandewiele/RMG-Py,enochd/RMG-Py,chatelak/RMG-Py,enochd/RMG-Py,nyee/RMG-Py,nickvandewiele/RMG-Py,KEHANG/RMG-Py,chatelak/RMG-Py,faribas/RMG-Py,faribas/RMG-Py,comocheng/RMG-Py,KEHANG/RMG-Py,pierrelb/RMG-Py