ENH: Add volume and discrete source space creation support
ENH: Discrete source space support
STY: Minor fixes
ENH: Clearer input arguments
FIX: Remove redundant code
ENH: Cleaner API without pos as str
ENH: Safer testing
ENH: Use float32 for slight memory savings
Merge pull request #781 from Eric89GXL/vol-src

ENH: Add volume and discrete source space creation support