Update v0.88 - Minor update

Hi just a small update today, added some recommended additions to the game such as:

You can now group units using SHIFT + 1-9 and then recall the group using the corresponding number.

There is now a guard button so that you can tell a group of units to guard another.

Turret hardpoints have now been added to the heavy walls.

You can now zoom in and out using the Q and Z on the keyboard, handy if you are building a structure.

Safety doors shouldn't cause units to explode when they walk over it.

Cheers Rob