Add Math functions

We should add native Maths functions for the most common operations:

• abs(x) returns the absolute value of the x
• exp(x) returns the value of e (the base of natural logarithms) raised to the power of x
• log(x) returns the natural logarithm (base e) of x
• log10(x) returns the base-10 logarithm of x
• round(x) returns the closest integer to x

Additional information for newcomers:

The new functions should be put in a new class MathFcts similar to TimeFcts.
The MathsFcts.all() method should be called in SystemKeyspace.functions
The unit tests for the functions should be put in a MathFctsTest similar to TimeFctsTest