fixed minor typos and unnecessary functions; added missing condition

[prankuragarwal]

• src/main/java/de/vill/conversion/ConvertTypeLevel.java : While converting, the actual value was getting lost and hence removed this

• src/main/java/de/vill/model/Attribute.java: added missing handling for Integers

• src/main/java/de/vill/model/constraint/ExpressionConstraint.java: Needed this during transformations for my thesis

• src/main/java/de/vill/model/expression/CeilAggregateFunctionExpression.java: typo

• src/main/java/de/vill/model/expression/FloorAggregateFunctionExpression.java: typo