Allow time step to increase after shrinking.

This addresses issue #3764.
small style modifications refs #3765 #3764
Add test for regrowing DT with ConstantDT refs #3765 #3764
Merge pull request #3855 from friedmud/constantDtRegrow_3764

Constant dt regrow
Merge commit 'b44aaa74e83ecfd9d6e772d94b8d35064ae014ea'