text: Title: [ENH] Expose sparse library functions
  Body: XArray supports the [Sparse](https://github.com/pydata/sparse) package but doesn't expose the functions to convert to/from sparse objects. These functions could be nicely packaged in pyjanitor to do so:
  
  ```python
  import sparse
  
  @register_xarray_dataarray_method
  def to_scipy_sparse(
      da: xr.DataArray,
  ) -> xr.DataArray:
      if isinstance(da.data, sparse.COO):
          return xr.apply_ufunc(sparse.COO.to_scipy_sparse, da)
      return da
  
  
  @register_xarray_dataarray_method
  def todense(
      da: xr.DataArray,
  ) -> xr.DataArray:
      if isinstance(da.data, sparse.COO):
          return xr.apply_ufunc(sparse.COO.todense, da)
      return da
  
  
  @register_xarray_dataarray_method
  def tocsc(
      da: xr.DataArray,
  ) -> xr.DataArray:
      if isinstance(da.data, sparse.COO):
          return xr.apply_ufunc(sparse.COO.tocsc, da)
      return da
  
  
  @register_xarray_dataarray_method
  def tocsr(
      da: xr.DataArray,
  ) -> xr.DataArray:
      if isinstance(da.data, sparse.COO):
          return xr.apply_ufunc(sparse.COO.tocsr, da)
      return da
  
  
  @register_xarray_dataarray_method
  def tosparse(
      da: xr.DataArray,
  ) -> xr.DataArray:
      if isinstance(da.data, sparse.COO):
          return da
      return xr.apply_ufunc(sparse.COO, da)
  ```