Rashdf

Features
* `rashdf` command-line interface
* `structures` method implemented

Fixes an issue with `rashdf.utils.convert_ras_hdf_string`. The already-existing function `rashdf.utils.parse_duration` wasn't getting used as intended to parse RAS HDF time duration strings. This update ensures that RAS HDF attribute string values in the format `HH:MM:SS` are converted to `datetime.timedelta` objects.

Features
* Initial classes for `RasHdf`, `RasGeomHdf`, `RasPlanHdf` as subclasses of `h5py.File`
* Methods implemented:
* `projection`: return the projection as a pyproj `CRS` object
* `mesh_area_names`: return the list of 2D mesh area names
* `RasGeomHdf` methods returning `GeoDataFrame` objects:
* `mesh_areas`: 2D flow area perimeter polygons
* `mesh_cell_polygons`: 2D flow area cells as polygons
* `mesh_cell_points`: 2D flow area cell centers
* `mesh_cell_faces`: 2D flow area cell faces as lines
* `bc_lines`: 2D boundary condition lines
* `breaklines`
* `refinement_regions`
* Methods to return dictionaries of attributes (i.e., metadata) for certain HDF groups:
* `RasGeomHdf`:
* `get_geom_attrs`
* `get_geom_structures_attrs`
* `get_geom_2d_flow_area_attrs`
* `RasPlanHdf`:
* `get_plan_info_attrs`
* `get_plan_param_attrs`
* `get_meteorology_precip_attrs`
* `get_results_unsteady_attrs`
* `get_results_unsteady_summary_attrs`
* `get_results_volume_accounting_attrs`

Features
* Initial classes for `RasHdf`, `RasGeomHdf`, `RasPlanHdf` as subclasses of `h5py.File`
* Methods implemented:
* `projection`: return the projection as a pyproj `CRS` object
* `mesh_area_names`: return the list of 2D mesh area names
* Methods returning `GeoDataFrame` objects:
* `mesh_areas`: 2D flow area perimeter polygons
* `mesh_cell_polygons`: 2D flow area cells as polygons
* `mesh_cell_points`: 2D flow area cell centers
* `mesh_cell_faces`: 2D flow area cell faces as lines