[1D] Reduce default minimum grid size

The new value is OK even for high pressure flames - it is less than the mean
free path in air at 100 bar and 300 K.

src/oneD/refine.cpp

@@ -9,7 +9,7 @@ namespace Cantera
 Refiner::Refiner(Domain1D& domain) :
     m_ratio(10.0), m_slope(0.8), m_curve(0.8), m_prune(-0.001),
     m_min_range(0.01), m_domain(&domain), m_npmax(3000),
-    m_gridmin(5e-6)
+    m_gridmin(1e-10)
 {
     m_nv = m_domain->nComponents();
     m_active.resize(m_nv, true);