MNT: Refactor constants (comments welcome)

closes #209 
`constants.py` was in the root of the `skxray` project.  I refactored it into a few files and put it in its own `constants` module.  The reason `xrf.py` looks like `constants.py` is because it used to be in `constants.py`.

Old structure:
```
skxray
  ↳ constants.py
```

New structure:
```
skxray
  ↳constants
     ↳ __init__.py
     ↳ api.py
     ↳ basic.py
     ↳ xrf.py
     ↳ xrs.py
```