ENH: add stringOps::toScalar and dictionary #eval directive

- the #eval directive is similar to the #calc directive, but for evaluating
  string expressions into scalar values. It uses an internal parser for
  the evaluation instead of dynamic code compilation. This can make it
  more suitable for 'quick' evaluations.

  The evaluation supports the following:
    - operations:  - + * /
    - functions:  exp, log, log10, pow, sqrt, cbrt, sqr, mag, magSqr
    - trigonometric:  sin, cos, tan, asin, acos, atan, atan2, hypot
    - hyperbolic:  sinh, cosh, tanh
    - conversions:  degToRad, radToDeg
    - constants:  pi()
    - misc: rand(), rand(seed)