[Bug fix] Change the default expression string for derived variables

[anagainaru]

When I try to bpls on an aca file (in Norbert's branch) I am getting seg fault

$ ./bin/bpls -l run-488.aca --show-derived
  double   ckpt.bp/U      {8, 66, 66, 66} = 0.0964156 / 1
Segmentation fault: 11

This is because bpls checks the length of the expression string to decide if a variable is derived or not:

std::string ExprStr = fp->VariableExprStr(*variable);
if (ExprStr.size() > 0)
     // do stuff

Normally we return an empty string so this is fine in most cases:

std::string BP5Reader::VariableExprStr(const VariableBase &Var)
{
    // check if this is a derived variable and return the correct string, else
    std::string noDerive("");
    return noDerive;
}

However, for some reason aca files use the default implementation in Engine.h which is wrongly returning a NULL pointer.