WIP: raw file splitting

Support for splitting of raw files. This is work in progress. Tests need to be added and automatic joining of split files when reading. 

Does anyone know how split files can be read in `mne_browse_raw`? I tried loading a file by loading the first part, but it didn't automatically load the other parts. 

closes #1194
```
if split_size[-2:] not in dict(MB=20, GB=30):
    raise ValueError ....
```