[FIX] models: improve rationale for the management of flag 'recompute' in context

When the context contains 'recompute': False, the recomputation was not even
prepared. Now both create() and write() prepare the recomputation by invoking
method modified(). The flag only controls whether method recompute() is invoked.
In addintion, the former flag 'no_store_function' was converted to the flag
'recompute', so that both create() and write() use the same flag.

Fixes #1456