Improve atoms collection

[beghelli]

Player should have the ability to collect atoms. This behavior is implemented now by just allowing the player to touch the atom and collect it.

We need to make this process a little bit harder so it is more challenging and involves a little bit more skill from the player.

The collection should only happen if the player is:

• facing the atom; if the atom touches the back or the sides of the player, it won't be collected;
• holding the SPACE key; it forces the player to be more active related to the collection process;
• have enough space in the recipient that should hold that atom;

To avoid spamming the SPACE key, holding it should consume energy, so players will only be holding it when they really need to.

If the player touches an atom without completing all necessary actions listed above, the collection should not happen. For now, let's not change anything related with both entities. The player and atom collision behavior should be implemented in another issue.