Load LambertConformal grids from netcdf

This was required for a user loading some NOAA data via OpenDAP.
-- specifically, from a url like 'http://nomads.ncdc.noaa.gov/thredds/dodsC/narrmonthly/198801/19880101/narrmon-a_221_19880101_0000_000.grb'
-- (N.B. not currently working during US budget arguments)
This data specifies a form with a single "standard parallel".  Although PROJ4 contains a separate coding for this, it seems adequate to simply use the "two standard parallels" form with a repeated value.  Doing it that way avoids having to change Cartopy as well to make the resulting cubes plottable.

NOTE: strictly, the original data also specifies a spherical ellipsoid with a specific radius.  This code doesn't yet support this, for the following reason ...
Whereas CF suggests that ellipsoid keys should be supported for all coord systems, we don't currently have  common code for this in our CF pyke rules.  So I'm making another PR to fix that + add in the spherical case.  Watch this space ....