Adds rectilinear-safe code, proper regridding, bug fixes

The main new feature of this code is proper regridding of fluid velocity data. There are two main parts of this:

• IB2d data is now automatically extended. No need to call wrap_flow
• Comsol data is now regridded to include the boundaries using extrapolation or, in the case of periodic BC, interpolation.

Because Comsol specifies data on the center of mesh cells, the second of these required Planktos to allow rectilinear mesh grids. The entire code base has now been made safe for this.

Bug fixes include Gaussian KDE plotting, save_pos_to_csv saving, and a few changes based on depreciations in the supporting libraries.