Hide Level unless language SecurityLevels is turned on

[dorchard]

Since that Level is now not really a Semiring (due to the weakening of properties to inequalities) it makes sense to not have it available (notice some Level tests are currently failing in dev-minor because of the change in properties e.g., frontend/tests/cases/positive/absorbLevels.gr). Some of these could be duplicated and converted to Sec or with the added language pragma.