Add type conversions to fix compiler warnings

The fix for issue #1 introduced new compiler warnings:

```
src/weighted_mean.c: In function ‘_weighted_mean_intermediate’:
src/weighted_mean.c:87:22: warning: assignment makes pointer from integer without a cast [enabled by default]
src/weighted_mean.c:88:25: warning: assignment makes pointer from integer without a cast [enabled by default]
```

This patch fixes those.