This lays the groundwork for the `math` module for datatable. In this PR I add - 30 math functions (all of them single-argument); - Documentation page which simply mentions which functions are ...

.. xfunction:: datatable.math.log10 :src: src/core/expr/funary/exponential.cc resolve_op_log10 :cvar: doc_math_log10 :signature: log10(x) Decimal (base-10) logarithm ...