run fieldgen and return a dataframe with the realizations

    Parameters
    ----------
    m : flopy.mbase
        a floy model instance
    num_reals : int
        number of realizations to generate
    struct_dict : dict
        key-value pairs of pyemu.GeoStruct instances and lists of prefix strings.  Example: {gs1:['hk','ss']}
    cwd : str
        working director where to execute fieldgen.  If None, m.model_ws is used.  Default is None

    Returns
    -------
    df : pandas.DataFrame
        a dataframe of realizations.  Columns are named to include structure, prefix and realization number.  Index
        includes i-j position

    Note
    ----
    only Ordinary kriging is supported
    only a single zone is supported

def run_fieldgen(m,num_reals,struct_dict,cwd=None):
    """run fieldgen and return a dataframe with the realizations

    Parameters
    ----------
    m : flopy.mbase
        a floy model instance
    num_reals : int
        number of realizations to generate
    struct_dict : dict
        key-value pairs of pyemu.GeoStruct instances and lists of prefix strings.  Example: {gs1:['hk','ss']}
    cwd : str
        working director where to execute fieldgen.  If None, m.model_ws is used.  Default is None

    Returns
    -------
    df : pandas.DataFrame
        a dataframe of realizations.  Columns are named to include structure, prefix and realization number.  Index
        includes i-j position

    Note
    ----
    only Ordinary kriging is supported
    only a single zone is supported


    """
    if cwd is None:
        cwd = m.model_ws
    set_file = os.path.join(cwd,"settings.fig")
    if not os.path.exists(set_file):
        print("writing ",set_file)
        with open(set_file,'w') as f:
            f.write("date=dd/mm/yyyy\ncolrow=no\n")

    m.sr.write_gridSpec(os.path.join(cwd,"grid.spc"))

    np.savetxt(os.path.join(cwd,"zone.dat"),np.ones((m.nrow,m.ncol),dtype=np.int),fmt="%2d")
    arrs = {}
    for struct,prefixes in struct_dict.items():
        args = ["grid.spc",'']

        print(struct)
        struct.to_struct_file(os.path.join(cwd,"pyemu_struct.dat"))

        args.append("zone.dat")
        args.append("f")
        args.append("pyemu_struct.dat")
        args.append(struct.name)
        args.append("o")
        args.append("10")
        args.append(num_reals)
        for prefix in prefixes:
            prefix_args = list(args)
            prefix_args.append(prefix)
            prefix_args.append("f")
            prefix_args.append(1.0)
            prefix_args.append('')
            rsp_file = "fieldgen_{0}.in".format(prefix)
            with open(os.path.join(cwd,rsp_file),'w') as f:
                for arg in prefix_args:
                    f.write(str(arg)+'\n')
            pyemu.os_utils.run("fieldgen <{0} >{1}".format(rsp_file,rsp_file.replace(".in",".stdout")),cwd=cwd)
            real_files = ["{0}{1}.ref".format(prefix,i+1) for i in range(num_reals)]

            for real_file in real_files:
                assert os.path.exists(os.path.join(cwd,real_file)),"missing realization file: "+real_file
                vals = []
                with open(os.path.join(cwd,real_file),'r') as f:
                    [vals.extend(line.strip().split()) for line in f]
                arr = np.array(vals,dtype=np.float)#.reshape(m.nrow,m.ncol)
                real_num = int(real_file.split('.')[0].replace(prefix,''))
                arrs["{0}_{1}_{2}".format(struct.name,prefix,real_num)] = arr
        ij = []
        for i in range(m.nrow):
            for j in range(m.ncol):
                ij.append("{0}_{1}".format(i,j))
        df = pd.DataFrame(arrs,index=ij)
        return df