This function constructs a string to be parsed when solver
is set to cplexapi.
optionsCplexAPISingle(name, vector)string, name of the cplexapi function to call to
implement the option.
a named vector, contains the argument names and
values of the options. The env argument in the
cplexapi documentation should always be omitted.
string, the command to be evaluated to implement a single option.