Add multi file structure

[felixbur]

Makes sense to hide functions from package users that would like to use this as a software library

• Switch to multi file structure
• Select which part of the code should be public
• Easier to refactor code later

spmath/__init__.py
spmath/core/__init__.py  # empty file
spmath/core/api.py       # was spmath.py before

• spmath/__init__.py contains

from spmath.core.api import from_db
from spmath.core.api import to_db