Extend Density unit Pound Mass Per Cubic Inch abbreviation list

[hailstorm75]

Is your feature request related to a problem? Please describe.
The abbreviation list of Pound Mass Per Cubic Inch should have an additional entry.

Describe the solution you'd like
Extend the abbreviation list with "lbm/in^3"

Describe alternatives you've considered
Use the UnitAbbreviationsCache for now:

var cache = UnitAbbreviationsCache.CreateDefault();
cache.MapUnitToAbbreviation<DensityUnit>(DensityUnit.PoundPerCubicInch, "lbm/in^3");

Additional context
Discovered when processing density values provided by Siemens Solid Edge.