Ability to merge/unmerge units

Units being able to merge with another or same type of unit, resulting in a different unit.
Said unit being able to unmerge into the original units.
Suggested uses; cavalry (mounting & dismounting), Viscariods

##### ADDITIONAL INFORMATION #####
The following is the summary and additional information from duplicate bug:606:

You choose one unit to "attack" a friendly unit of the same type, and they play an animation, then a new unit is created. No thoughts on the how to implement the coding though.

I do have suspicions that this is too hard to make work, and maybe there are more efficient processes to achieve this.

-MT

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Perhaps link this to the weapon.

Here are ideas for new tags and what you could use, doubt it is of any use though :P

1. Check AffectEnemy=Y/N to see if it works, just in case this can be fired on an enemy.
2. Check Affects=SOMEUNIT and see if the two types of unit are affected by the weapon
3. Destroy the two units
4. Use Delay=N1,N2 for the time delay, N1 being the delay for the anim to start after the initial units were removed, N2 being the delay for the actual spawning of the units to commence. Of course, 0,0 would be for no delays
5. Use MergeAnim=SOMEANIM to decide what animation to play at this stage
6. Use UnitResult=FINISHEDMERGINGUNIT to decide which unit to spawn at the end