Use of scipy.optimize.curve_fit() for 2 tiles in blockwise

[adebardo]

Context

In terms of the RANSAC steps specifically: Maybe we don't need to run RANSAC below X number of tiles (where does the number 5 come from?), because there aren't enough tiles to detect outliers anyway (and with such large tile sizes, there shouldn't be any outliers). We could move the RANSAC to trigger only above a certain number of tiles (and for the rest, default to a simple 2D plan fit with scipy.optimize.curve_fit()). This could also be the occasion to make the RANSAC step optional altogether to keep modularity for users.