finish a simple correlation measurement

Correlation is calculated in a linear manner from the end of the chain
back propagate to the begining.
upgrade to ARPACK solver

finish a more robust solver by calling LAPACK and ARPACK.
Correlation Measurement, Tensor Convensions

change tensor convensions. add comments of tensor network. rewrite
MAKE_TS, NEW_TX. correct two bugs: (1) locate max mode is rewritten,
previously imaginary-part sensitivity is removed. (2) MPO tensor is
split to A and B types, as they are not symmetric which previously
assumed.

Also finish the correlation measurement subroutine. can measure the
correlation function recursively on the MPS generated by fDMRG.
 Add tensor rescaling

rescale the tensor each step after update, the rescaling factor is
saved by logorithm in H, instead of tracking Tval, tracking the energy
H now.
Add aviod crossing option

modified the vertex tensor based on the loop gas model
Add entropy measuring

also change the naming system
update TENSOR.f90
Remove MAX_LEN feature, upgrade TENSOR.f90

remove MAX_LEN, restore LEN in MODEL and stop passing it to
subroutines. upgrade TENSOR.f90 to deal with tensor array I/O.
fix the segfault bug, add data collection

fix the segfualt bug in ANNEAL1. when dumping W to RESID, the W index
starts from 0, which exceed the lower bound of RESID. fix it by +1.
also apply the +1 index convension to all the other
tensor-vector/matrix interface.

add data collection subroutine. write filenaming system.

put SWEEP into MODEL

SHOW_DMRG_STATUS now broadcast the SVD relative error

upgrade TENSOR.f90 to detect allocation status in TEN_PROD in order to
prevent segfualt when fail to allocate space.
Merge pull request #6 from EverettYou/fDMRG-PBC

F dmrg pbc