allow blocked factories to build ("canBuildWhileBlock=true")

[knorke]

Atm a factory does not create a unit when they are blocked by a unit or feature.
It would be nice if this could be overwritten by a new tag like canBuildWhileBlocked=true
(How/If the unit gets out of the factory would be up to the modder.)

One use might be flying factories:
http://springrts.com/phpbb/viewtopic.php?f=21&t=28154
and maybe other taradiddle.