Evidently, the functionality of most math functions is the same as provided by the `expr_unaryop` module, so it makes no sense to keep them separate, duplicating the code. In this PR, all expr_math ...

.. 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 ...