This operator implements a segregation transport flux across an interface.
Parameters: 
Tr m
is a field or function
is a field or function
Tr is a variable
m is a variable
Note that both sides of an interface should have the same segregation flux, but with opposite signs. It is recommended that the same form be used for both, but with different -solvesFor and -sign.
A useful rule for determining the sign of each operator is that the sign should be chosen to reduce the -solvesFor field if the -solvesFor field increases.