Towards less eager imports

Defer imports in gpaw/__init__.py.

It's tricky to intercept the imports through the CLI back-and-forth, and that shows. We could consider switching to click.

Anway: In current master, importing GPAW increases the number of loaded modules like this:

askhl@alberich:~$ python3 count_imports.py 
Imports from gpaw module 55 -> 877

With this branch, the number is:

askhl@alberich:~$ python3 count_imports.py 
Imports from gpaw module 55 -> 74

The script for counting the imports is:

import sys
mods = set(sys.modules)
import gpaw
mods2 = set(sys.modules)

print('Imports from gpaw module', f'{len(mods)} -> {len(mods2)}')
# print(' '.join(sorted(mods2 - mods)))