clear up our help text (including defaults) (#11)

gridded_glm/libexec/_minute_gridder.py

 #!/usr/bin/env python3
 # Based on https://github.com/deeplycloudy/glmtools/blob/master/examples/grid/make_GLM_grids.py
 
-parse_desc = """Create one minute grids of GLM flash data.
+parse_desc = """Create one minute NetCDF4 grids (and, optionally, AWIPS-compatible tiles) from GLM flash data.
 """
 
 import numpy as np
@@ -28,18 +28,17 @@ def create_parser():
     import argparse
     parser = argparse.ArgumentParser(description=parse_desc)
     parser.add_argument('-v', '--verbose', dest='verbosity', action="count", default=0,
-                        help='each occurrence increases verbosity 1 level through ERROR-WARNING-INFO-DEBUG (default ERROR)')
+                        help='each occurrence increases verbosity 1 level through ERROR-WARNING-INFO-DEBUG (default: ERROR)')
     parser.add_argument('-l', '--log', dest="log_fn", default=None,
-                        help="specify the log filename")
-    # from Requirements: "Output is Gridded GLM in the native glmtools NetCDF4 format, with a user option to produce AWIPS-compatible NetCDF tiles as described below"
-    parser.add_argument('-o', '--output-dir', metavar='output directory',
-                        default=os.getcwd())
-    parser.add_argument('--goes-sector', default="full",
-                        help="One of [full|conus|meso]. "
-                             "If sector is meso, ctr_lon and ctr_lat "
-                             "are interpreted as the ctr_x and ctr_y of the fixed grid.")
+                        help="specify a log filename. (default: print to screen).")
+    parser.add_argument('-o', '--output-dir', metavar='OUTPUT_DIR',
+                        default=os.getcwd(), help="output directory (default: use current directory)")
+    parser.add_argument('--goes-sector', default="full", choices=['full', 'conus', 'meso'],
+                        help="If sector is meso, ctr_lon and ctr_lat "
+                             "are interpreted as the ctr_x and ctr_y of the fixed grid."
+                             "(default: full)")
     parser.add_argument("-t", "--create-tiles", default=False, action='store_true',
-                        help="create AWIPS-compatible tiles") # FIXME: improve this help text
+                        help="create AWIPS-compatible tiles (default: off)")
     parser.add_argument('--ctr-lat', metavar='latitude',
                         type=float, help='center latitude (required for meso)')
     parser.add_argument('--ctr-lon', metavar='longitude',