need to zero out when resizing
prep for 0.5.4
bump version in README
add comment for #209
check for valid coefficients, fixes #210
fix CPLEX sos test
Merge pull request #211 from JuliaOpt/nancheck

check for valid coefficients, fixes #210
tidy GenericAffExpr operators
definition is only safe if applied to instance of the same type
try to avoid try
Merge pull request #206 from JuliaOpt/no-try-catch

RFC: Try to avoid try
clear internal model if we change something we're not supposed to