ignore build files
Use PBC and switch to loop gas model

Periodic boundary condition by defining environment tensor with 4 legs,
closing the boundary. Switch the vertex tensor from FPL to loop gas,
adding temperature dependence.
recover developer, ignore nb file
fix the symmtry of SYSVD on X

also rewrite INV_SQRT by Schur decomposition to improve numerical
robustness
vectorized power iteration

upgrade annealing solver, from tensor to vector operation, speed much
faster, but still have the problem that power iteration may not find
the max state, stuck to metastable state.
Upgrade annealing solver to Arnoldi

Use Arnoldi iteration to anneal the ground state. accurate, robust and
fast.
single-site sweeping

Works really bad, state not improved, Tval not uniform, and result not
converging.
two-site sweep fDMRG

great! this works much better than single-site sweep, the Tval seems to
converge. Still need to work on the boundary effect.
Pack iDMRG and fDMRG steps, deal with boundary

Pack iDMRG and fDMRG steps to subroutines, clear up the kernel, add a
show DMRG status routine to print data. By branching according to L in
fDMRG, the boundary issue has been handled. The boundary tensor is
restarted under each sweep, such that spontaneous symmetry broken can
take place.
ignore *.dat and *.ten
clear up data files
Merge pull request #4 from EverettYou/fDMRG-PBC-LG

F dmrg pbc lg