FDS Source: add TEST_NEW_CHAR_MODEL

[rmcdermo]

This PR adds an experimental char model ( invoked with TEST_NEW_CHAR_MODEL=T on MISC ) that starts the X_O2 profile at the CHAR_FRONT, defined as the solid cell where the CHAR density exceeds the ASH density. This basically assumes no mass transfer limitation through the ASH layer and provides a more physically reasonable picture of the O2 profile superimposed on the CHAR front, as shown below.