Add some math functions

[vicllo]

Hello ! This pull request is a first add of some maths functions written in Ocaml. Do not hesitate to tell me if anything needs to be changed !

• Fibonacci sequence calculator using recursion
• Absolute value
• Absolute sum of digits of an array
• Minimum value search in an array
• Maximum value seacrch in an array
• Ceil value of a float
• Recursive power calculator