BUG: TypeError being raised by map_to_grid.

map_to_grid would raise a TypeError when multiple radar objects were passed
and the fields parameters was None.

Bug reported by Marcus van Lier-Walqui, 2013/11/21
Merge pull request #106 from kmuehlbauer/master

ENH: IO/read_gamic, phase_proc/new solver
Merge branch 'master' of https://github.com/ARM-DOE/pyart
DOC: updated README.rst and INSTALL.rst
MAINT: added sigmet_task_name to metadata of radar object returned from read_sigmet
BUG: time ordering sigmet files does not ignore bad rays
ENH: Better support for vertial pointing scans

* is_vpt function for checking if a radar object is a VPT scan.
* to_vpt function for converting a radar object to a VPT scan.
* plot_vpt method on RadarDisplay class for plotting VPT scans.
Merge pull request #108 from jjhelmus/master

Support for vertical pointing scans and bug fixes
ENH: reading MDV files from file obj
BUG: None is allowed cax parameter in plot_colormap of GridMapDisplay
MAINT: updated grid example for new plot_colorbar syntax
Merge pull request #110 from jjhelmus/master

BUG: GridMapDisplay.plot_colorbar should allow cax=None.
ENH: reverse_xaxis parameter added to plot_rhi

RadarDisplay object's plot_rhi method now supports a reverse_xaxis
parameter when will change the east-west direction of the plot to a
west-east direction.  The default of None reverses the direction only
when only negative distances are present.

Closes #111
Merge pull request #113 from jjhelmus/master

Enhancements and bug fixes based on issues
BUG: masked values no longer appear in grids

Masked values no longer are allowed to 'leak' through into Grids which are
produced from the `map_to_grid` function when **copy_field_data** is True.
The documentation for the `map_to_grid` function has been updated to better
explain the **copy_field_data** parameter and provides a method for preventing
masked values from leaking through to the grid when **copy_field_data** is
False.

Closes #105
ENH: radar_n_ keys in Grid metadata

Grid objects produced from the `grid_from_radars` function have
radar_n_lat, radar_n_lon, radar_n_alt, radar_n_instrument_name
keys in the metadata dictionary where n is the radar number
(starting from 0).

This addressed some on the concerns in issue #107.
WIP better default qrf_func
ENH: better default roi_func in map_to_grid

* Renamed `qrf_func` to `roi_func` in map_to_grid.
* Three build in `roi_func` build in, 'constant', 'dist' and 'dist_beam',
  'dist_beam' is the default for backwards compatibility.
* The 'dist' and 'dist_beam' built in RoI functions now take into
  account the offset between the nearest radar and the grid when
  computing the RoI.
* grid_shape and grid_limits are now required parameters to the
  map_to_grid function.
* grid_shape, grid_limits, and roi_functions have dimension ordered as
  z, y, x, which matches the output grid shape. Internal this order is also
  used.
* Better documentation for all of the above.
DOC: fixed map_to_grid docstring
EXAMPLES: updated mapping examples