Write generic function to generate a sets file for profiles

[petebachant]

More specific example:

def gen_sets_file():
    """Generate ``sets`` file for post-processing."""
    # Input parameters
    setformat = "raw"
    interpscheme = "cellPoint"
    fields = ["UMean", "UPrime2Mean", "kMean"]
    x = 1.0
    ymax = 1.5
    ymin = -1.5
    ny = 51
    z_H_max = 1.25
    z_H_min = -1.25
    nz = 19
    H = 0.807
    zmax = z_H_max*H
    zmin = z_H_min*H
    z_array = np.linspace(zmin, zmax, nz)
    txt = "\ntype sets;\n"
    txt +='libs ("libsampling.so");\n'
    txt += "setFormat " + setformat + ";\n"
    txt += "interpolationScheme " + interpscheme + ";\n\n"
    txt += "sets \n ( \n"
    for z in z_array:
        # Fix interpolation issues if directly on a face
        if z == 0.0:
            z += 1e-5
        txt += "    " + "profile_" + str(z) + "\n"
        txt += "    { \n"
        txt += "        type        uniform; \n"
        txt += "        axis        y; \n"
        txt += "        start       (" + str(x) + " " + str(ymin) + " " \
            + str(z) + ");\n"
        txt += "        end         (" + str(x) + " " + str(ymax) + " " \
            + str(z) + ");\n"
        txt += "        nPoints     " + str(ny) + ";\n    }\n\n"
    txt += ");\n\n"
    txt += "fields \n(\n"
    for field in fields:
        txt += "    " + field + "\n"
    txt += "); \n\n"
    txt += "//\
     *********************************************************************** //\
     \n"
    with open("system/sets", "w") as f:
        f.write(txt)