Fix base acceleration computation without gravity
Fix base acceleration computation without gravity

Fix base acceleration computation without gravity effect by using `copy`
to copy the object instead of a simple assignment. This is cause simple
assignment assigns the address of the object instead of the object's
value.

galou/symoro,ELZo3/symoro,symoro/symoro,symoro/symoro,galou/symoro,ELZo3/symoro