Add a test for Poisson solver using FFT
Create test functions
Add a notebook for Poisson convergence
Do larger convergence study
Implement Ewald sum test
Update notebook
Use Z=3, add constants to global variables
Add a test for 2 Gaussian charges
Use alpha=30
Use _dp
Use alpha=5 and Ng=32
Refactor creation of Gaussian charges
Use gaussian_charges()
Add a test for NaCl
Add a test for distored NaCl
Print Ewald forces
Calculate forces using FFT
Use radial FFT
Print errors
Test the errors