fix: install with richdem even if it is not available

[mcflugen]

This pull request allows landlab to be installed even if the richdem package is not available, which, for now, means leaving out richdem for Python 3.10 on Windows. Users will be able to use the entirety of landlab except for the PriorityFloodFlowRouter.

We, ultimately, will want to fix richdem to work on all of landlab's supported platforms but this acts as a workaround until that time.

[mcflugen]

In addition to fixing the issue with richdem, I've also fixed a related issue with the bmi-topography package (as seen in #1382, and #1381). As part of a previous pull request, bmi-topography was mistakenly added as a landlab requirement—although bmi-topography is used by a notebook tutorial, it's not used by landlab. Moving it from the requirements file to the notebook requirements file fixed the landlab installation issue but moved the same issue to notebook installation. The problem being that bmi-topography requires rasterio but rasterio is not available from PyPI for some platforms (namely, Windows).

With this pull request, landlab can now be installed with either pip (getting dependencies from PyPI) or conda (getting dependencies from conda-forge) but the requirements for the notebooks need to be installed using conda (for Windows, anyway).