Pylandstats

- fill `NaN` values according to the metric in `MultiLandscape.compute_class_metrics_df`
- consistent imports so that there are no direct class/function calls, and no module abbreviations in the documentation
- zonal analysis with more general `masks` argument, which also accepts vector geometries (and replaces the now deprecated `masks_arr`)

- ensure numeric dtypes in metrics data frames
- `np.nan` nodata in img passed to `rasterio.plot.show` in `plot_landscape`

- more robust `loc` (with index and columns) to compute metrics data frames in `MultiLandscape`

- corrected `ZonalGridAnalysis` docstring
- corrected computation of `num_zone_cols` in `ZonalGridAnalysis`

- configured flake8 in setup.cfg and added flake8 test in travis.yml
- added `compute_zonal_statistics` method to `ZonalAnalysis` and aded `ZonalGridAnalysis` class
- Implemented the `effective_mesh_size` metric

- gradient -> zonal
- corrected cmap arg: default from rcParams, accept str
- updates in the docs (changed link to notebook + cleaned Makefile)