Bug: No control tower available at August 1909

[DrSuperGood]

The first aircraft, the Zeppelin LZ 6 is introduced on August 1909. However in order to use any aircraft one needs to build control towers at the airport stops. There are no control towers available at August 1909 so the aircraft cannot be used at all. In fact it is many years before the first control tower becomes available and aircraft can be used.

I understand that there might be problems with the assets for an early control tower. However until those are sorted out one must permit the construction of later period control towers so as to allow early air to be at all possible.

[jamespetts]

Thank you for the report - I have now fixed this, which fix should be available in the next nightly build.