Cleans up steepest descent initial potential hack

Cleans up steepest descent initial potential hack

Addressed Bug #1676, in which there was a hack to
the do_steep() function in minimize.cpp where the
initial value of s_min->epot was set to
s_try->epot + 1; this patch ensures that the first
step's s_min->epot value is the true minimum
potential energy value and adds explicit checks
for the first step where necessary to ensure that
the behavior of do_steep() is unchanged.

Change-Id: I19c64a4948dfa24805345d7e593397d12ad4cc48