Convert wall reaction parameters in EN_setflowunits

[aowis-becker]

EN_setflowunits() updates a project's unit system and already converts
unit-dependent hydraulic data, but wall reaction parameters are currently
left unchanged.

This changes the physical meaning of wall reactions when switching between
US customary and SI flow units. For example, a first-order wall coefficient
of -1 ft/day becomes -1 m/day instead of the equivalent -0.3048 m/day.

This change converts wall reaction parameters whenever the unit system
changes:

• first-order wall coefficients as length/time,
• zero-order wall coefficients as mass/area/time,
• the global wall reaction coefficient,
• per-pipe wall reaction coefficients, and
• the roughness/reaction correlation factor.

Two regression tests cover first-order and zero-order wall reactions. They
verify both the converted coefficient values and preservation of the
resulting water-quality solution across a US-to-SI unit change and back.