Anderson-Mixing Variable Cell Implemented

[akasharora123]

• Anderson-mixing variable cell implemented: Only iterate module is changed.
• I/O module is modified to print 40 characters long comments, specifically, "max (SCF + 100xStress) residuals" during iterations on screen.
• Error calculation step is changed in the AM iteration. The earlier version of the code for AM used to calculate error only for the systems having only 2 and 3 different monomer types. Now the error is generalized by taking the maximum of the whole residual array, which is a three-dimensional array of size N_hist x (residuals of one monomer) x N_monomers.