Ewald summation

The test prints:
```
$ ./test_ewald 
 a =   564.01999999999998      pm
 Ewald:     -860.95778244239625      kJ/mol
 Madelung:  -860.95778244239409      kJ/mol
 error:     8.32667268468867405E-016 a.u.
 a =   550.00000000000000      pm
 Ewald:     -882.90437900574591      kJ/mol
 Madelung:  -882.90437900574386      kJ/mol
 error:     7.77156117237609578E-016 a.u.
 a =   450.00000000000000      pm
 Ewald:     -1079.1053521181313      kJ/mol
 Madelung:  -1079.1053521181316      kJ/mol
 error:     5.55111512312578270E-017 a.u.
 a =   650.00000000000000      pm
 Ewald:     -747.07293608178031      kJ/mol
 Madelung:  -747.07293608178315      kJ/mol
 error:     1.11022302462515654E-015 a.u.
 a =   1000.0000000000000      pm
 Ewald:     -485.59740845315991      kJ/mol
 Madelung:  -485.59740845315918      kJ/mol
 error:     2.77555756156289135E-016 a.u.
```