fix(code_bmc.ml): fix mode index in variable name
fix(opensmt/smtsolvers): rename short_sat => nra_short_sat
feat(opensmt/smtsolvers): add --model option

When the result is SAT, it create a file <XXX.smt2.model> which
contains a model (satisfying assignment).
feat(opensmt/smtsolvers): add --model option