Logarithm and exponential function nodes (ln, log10, exp)

[Yaraslaut]

Problem

function.hpp offers integer powers, roots and pi. Test methods also use natural logarithm, decimal logarithm and exponential of a dimensionless expression — to linearise a measured quantity, to state a model curve, or to express a result on a logarithmic axis. Today a consumer has to declare each of them as an opaque operation, so the page renders them as a call and the trace marks them [inside not shown], although they are ordinary functions a method writes out.

Wanted

• formula::ln(expr), formula::log10(expr), formula::exp(expr) as function nodes, rendered in every dialect (ln x, \ln, \log_{10}, e^{x} / \exp) and traced step by step like sqrt.
• The operand must be dimensionless — a dimensioned argument is a compile error in the library's own words, as for addends that disagree.
• A general real power pow(base, exponentExpr) with both dimensionless, or a note on why it is out of scope.

Open points

• Exact evaluation. The results are irrational almost everywhere. Should checked_evaluate (always exact) refuse a formula that contains one, or round it at a declared precision, as rounded_sqrt does? checked_evaluate_si<double> can answer directly.
• Domain. ln/log10 of zero or a negative value should be DomainError, never NaN or -inf, under both representations.
• Absence. An absent operand stays absent, like every other node.

[christianparpart]

Implemented in #7 and released in 0.2.0: ln, log10 and exp as function nodes, exact where the value is rational and otherwise Inexact, and rounded_ln, rounded_log10 and rounded_exp for the correctly rounded decimal at declared places. The guide is docs/expressions.md, Logarithms and exponentials.