feat(opensmt): add abs function
fix(opensmt/parsers/smt2/CMakeLists.txt): dependency on smt2parser/lexer
fix(opensmt/parsers/smt2/CMakeLists.txt): suppress sign-compare warning
feat(opensmt/egraph/EgraphStore.C): remove sort check in mkEq

For now, we remove the requirement that

    "sort(LHS) = sort(RHS)"

. We still need to check LHS and RHS are Real/Int (or Int/Real).
tests(nra): add abs tests
feat(opensmt/egraph/EgraphStore): subst x * x with x^2
fix(realpaver_rp_interval): (-oo, 0] * (-oo, oo) = (-oo, oo)
fix(realpaver/rp_operator_sat): memory leak
feat(realpaver/rp_operator): show constraint when debugging
fix(realpaver/rp_interval): display_simple outputs to stderr
tests(realpaver): add realpaver tests using Google Test
fix(opensmt/egraph/EgraphStore.C): use cons properly when make mkPow
feat(realpaver): support hexadecimal float
feat(opensmt/parsers/smt2): support hexadecimal float
tests(nra): add hex_01 test
chore(tests/.gitignore): ignore *.model
fix(heuristic): avoid backtracking in heuristic unless required

fix(style)

fix(heuristic): avoid backtracking in heuristic unless required

fix(style)
feat(heuristics): checkpoint on implementing ode simulation

feat(heuristics): checkpoint on implementing ode simulation

feat(heuristics): checkpoint on implementing ode simulation

feat(heuristics): checkpoint on implementing ode simulation

feat(heuristics): checkpoint on implementing ode simulation

feat(heuristics): checkpoint on implementing ode simulation

feat(heuristics): checkpoint on implementing ode simulation

feat(heuristics): checkpoint on implementing ode simulation

feat(heuristics): checkpoint on implementing ode simulation

feat(heuristics): checkpoint on implementing ode simulation