In MS-SQLServer, the following functions return a value of the same
type as the given argument:
  - abs
  - ceiling
  - degrees
  - floor
  - radians
  - round.

Since ADQL requires that these functions return a DOUBLE
(i.e. a float in MS-SQLServer), it must be ensured that the given
argument is a float.

This commit partially fixes the Pull Request #16