Iconarray

This release brings an enhancement to the `iconarray.ind_from_latlon()` function which was not performant on the ICON-1 domain of MCH. The new implementation uses a BallTree to search and can also return the N nearest points, rather than just one. For more information see https://github.com/C2SM/iconarray/pull/59.

This release brings an enhancement to the `iconarray.open_dataset()` function, with support to filter by a single variable, irrespective of whether the data is GRIB or NETCDF. The standard xarray and cfgrib arguments can also now be passed to xarray/cfgrib via `iconarray.open_dataset()`.

There is also an additional function `iconarray.show_GRIB_shortnames()` which prints the variable names available to use to filter GRIB files with `iconarray.open_dataset()`. This function does not load the entire dataset so is relatively quick.

Tests are added for the new fitler functionality.