Made the linear extrapolation in g_bar non-default behavior
g_bar currently resorts to linear extrapolation of dH/dl if
it doesn't find covering foreign lambda deltaH values. This is
dangerous because the most likely case of this being used is when
the user forgets to set foreign lambdas.
This commit adds an option -extp to g_bar that the user must
explictily set to enable the linear extrapolation. If not, and
there is no foreign_lambda path, g_bar prints an error message
with details of what to do.
Change-Id: Ib39f964b4f43f79379f00a3331ea4dc88183ed63