Implement cuboid variable!

[Pablete1234]

Is it a region? is it a variable? is it a filter?
It's all of them!

This pr introduces a cuboid variable type:

<variables>
  <cuboid id="your_cuboid" min="1,1,1" max="2,2,2"/>
</variables>

This variable can be used in most places that accept a region or filter, but it is also mutable via actions just like other variables.

<set var="your_cuboid.min_x" value="-5"/>

Additionally these can be be used as an indexed variable (ie: like arrays):

<set var="your_cuboid" index="0" value="-5"/>

the "indexes" would be min_x = 0, min_y = 1, min_z = 2, max_x = 3, max_y =4, max_z = 5, but generally speaking you're better off just using the sub-variables ie your_cuboid.min_x instead.

One such of a time when addressing with indexes could be used would be copying, eg:

<variables>
  <cuboid id="idx" scope="match"/>
  <cuboid id="cuboid_a" min="1,1,1" max="2,2,2"/>
  <cuboid id="cuboid_b" min="1,1,1" max="2,2,2"/>
</variables>
<actions>
  <action id="copy-a-into-b">
    <set var="idx" value="0"/>
    <repeat times="6">
      <set var="cuboid_b" index="idx" value="cuboid_a[idx]"/>
    </repeat>
  </action>
</actions>