Add functions to express dimensioned quantities.

Previously, MCAD libraries that dealt with absolutely-sized
artifacts (for example, nuts_and_bolts.scad) either implicitly worked in
millimeters or used a set of multiplicative constants defined in
units.scad (and one constant in constants.scad).

This change introduces a set of scaled functions in units.scad that help
creating objects that are specified in mixed dimensions:

        mm(q): millimeters
        cm(q): centimeters
        dm(q):  decimeters
         m(q):      meters
      inch(q):      inches
       mil(q):        mils (0.001 inch)

These functions convert from the indicated quantity to OpenSCAD's
internal units.  By default, they are set up so that

    1 /*OpenSCAD unit*/ == mm(1),

but, this can be switched by setting the variable MCAD_BASE_UNIT before
including units.scad.  MCAD_BASE_UNIT is a multiplicative scale factor
that determines the correspondence between MCAD millimeters and OpenSCAD
units.  For example:

    // a.scad
    include <MCAD/units.scad>
    echo(cm(1)); // Outputs 10

    // b.scad
    include <MCAD/units.scad>
    MCAD_BASE_UNIT = MCAD_BASE_UNIT_CM;
    echo(cm(1)); // Outputs 1

Every place in MCAD that was previously using the scale constants from
units.scad and constants.scad has been converted to use the new scale
functions, including the definition of MCAD_EPSILON (now 0.01 mm).

The scale constants have been kept for backwards-compatibility, but are
now defined in terms of the scale functions.

A small suite of tests has been added in the test/ subdirectory of the
project root.  The test suite can be invoked using 'make test' from the
top project directory.