feat(SWF): Introduce Surface Water Flow (SWF) Model

[langevin-usgs]

This PR introduces a first cut at a new Surface Water Flow (SWF) Model. The following are a few main points:

• This first version is based on diffusive wave routing through a connected network of one-dimensional channels.
• A channel cross section package can be used to specify channel geometries. If not specified, channel cross sections are represented as rectangular.
• Flow through the one-dimensional channel network is solved using the Iterative Model Solution.
• A SWF-GWF exchange is included, which can be used to couple a SWF Model and a GWF Model. Conductance formulations for flow between a surface water cell and a groundwater cell are presently limited to a user-specified conductance.
• There are presently three different boundary packages that can be specified for the network. These packages include the constant head (CHD), specified inflow (FLW), and zero-depth-gradient (ZDG) packages.
• Input instructions for the SWF Model and its packages are included in mf6io.pdf.

This is a rough first cut at the SWF Model, and there is no guarantee that it will be released. It is going into develop for further consideration, but will likely be left out of any official MODFLOW 6 releases until it has been thoroughly vetted and tested. The input and output format is likely to change without notice.