Henry's Law coefficients needs refactoring #153
Description
Current implemenbtation of Henry's Law coefficients by means of a subroutine called by GF is problematic. For one, it relies on field names for retrieving the coefficients. Such coefficients need to be provided as attributes of each tracer field, with the component that owns the tracer having the responsibility to set the attributes. GF should blindly retrieve the qttribute coefficients and perform the calculations.