Added surface area of a cuboid (#7293)

* Added surface area of a cuboid

Added surface area of a cuboid according to the formula:
S = 2 * (ab + ac + bc)

* Removed extra white space

* Added test for cuboid surface area

* fixed syntax error

* Removed extra space

* Added tests for cuboid surface area that should fail

I have added tests for cuboid surface area where one of the parameters is invalid. These should fail.