`Grid` and `MultiAxis` related naming

While working on the `MultiAxis` implementation I stumbled over a few potentially suboptimal names

1. `global bin` and `local bins`
- `bin` is actually an `index` - we could call it that way
- `global` / `local` is a loaded term in ACTS and usually refers to different coordinate systems
- my suggestion here would be something like `flat index` or `linear index` and `multi index`
2. for `Axis` we use `getX` while for `Grid` we use `x`
3. `lower left edge` / `upper right edge`
- I believe this should rather be `corner`
4. `position` vs `point`