Add AutoMPO concat function for local operators

[ryanlevy]

Addressing #219 and this forum post, this provides a concatenation function for local operators and AutoMPO

Usage:

auto ampo = AutoMPO(sites);

//loop in a function
for(auto n: range1(N))
   ampo.concat("Sz",n);
ampo.combine(); //done concating, does the += step

//For coefficients and a function
Real J = -1.7;
//same as ampo += J,"Sz",1,"Sz",2,"Sz",3;
ampo.concat(J);
for(auto n: range1(3))
    ampo.concat("Sz",n);
ampo.combine();

//don't like the comma operator? This works too
//same as ampo += 3.14,"Nup",2,"Ndn",4,"Nupdn",7";
ampo.concat(3.14,"Nup",2)
    .concat("Ndn",4,"Nupdn",7) //takes any number of arguments
    .combine();

Caveat: only one concat sequence is allowed at a time.

I'm not 100% in love with the names, so please feel free to suggest alternatives.

[dkweiss31]

Thank you!