phiH calculation should use initial electron momentum, not final

The scattered electron momentum is currently being used in phiH calculations; it should be the initial beam electron momentum. Note: this does not make any difference, since
```
q x l / |q x l| == q x l' / |q x l'|
```
but it's better to be consistent with Trento conventions as much as possible.

NOTE: this should be fixed in #192 too