Plotting Package scripts for the GST Algorithms

[arango]

Added several new scripts to facilitate the plotting of the Generalized Stability Theory (GST) algorithms using ROMS Plotting Package:

    plt_afte.sh      Adjoint Finite Time Eigenmodes (AFT_EIGENMODES)
    plt_fte.sh       Finite Time Eigenmodes (FT_EIGENMODES)
    plt_fsv.sh       Forcing Singular Vectors (FORCING_SV)
    plt_op.sh        Optimal Perturbations (OPT_PERTURBATION)
    plt_so.sh        Stochastic Optimals (SO_SEMI)

It can plot a single or multiple NetCDF files containing the eigenvectors. It is currently configured for the double gyre test case. However, Users can modify these scripts easily for the own application.