isAllowedOnFeature

Check: Is this improvement allowed on a given terrain feature?

Background: This not used for e.g. a lumbermill - it derives the right to be placed on forest from terrainsCanBeBuiltOn. Other improvements may be candidates without fulfilling the terrainsCanBeBuiltOn check - e.g. they are listed by a resource as 'their' improvement. I such cases, the 'unbuildable' property of the Terrain feature might prevent the improvement, so this check is done in conjunction - for the user, success means he does not need to remove a terrain feature, thus the unique name.