Replace GRAV_ENGLISH_LBM

Desired capability or behavior.

GRAV_ENGLISH_LBM is a value in constants.py that describes the conversion lbm/lbf at sea level (which is 1). This clearly is not applicable in any other scenario, such as another planet.

This constant is typically used in our mass components, as the legacy codes computed everything in terms of forces. In many cases we should just directly change the component to use mass. In some components, this doesn't make sense and we need to keep using forces, but converted from mass using actual gravitational acceleration ((lbf = lbm * g_current / g_earth). You need the ratio because in the english system F = M * a / g_earth. An example of a component that needs to use forces is the FLOPS detailed wing equations, which size the wings to resist bending moments including from the engine pods - the force the engine pods exert on the wing is reduced in lower gravity, so we need to account for that.

Suggested implementation.

No response