Minor updates in raster resampling section

[florisvdh]

Some minor suggestions, please have a look to judge what is actually better 😉. It's a well understandable and didactic section @Nowosad 👍!

Especially check the small additional explanations in order to distinguish cubic interpolation and cubic spline interpolation - since you already added 'splines (polynomial functions)' to explain the latter. Main source is Wikipedia (and see also dsp stackexchange) since it's not explained in terra::resample(). I didn't dig further behind the scenes in terra::resample(), so best double check. BTW according to Wikipedia, there's also bicubic interpolation, used for 2D interpolation, which could be the algorithm used here (unsure).

Beside that, maybe check how the terms 'grid' and 'raster' are used in the book. I assume each has its own meaning, but better don't unnecessarily mix them, since it then seems intended and may be confusing. Cf. below extract from chapter 2, where (I think) the 'grid' term is introduced and is regarded a more general concept than 'raster'. In this PR, 'grid' is replaced by 'raster'.

The spatial raster data model represents the world with the continuous grid of cells (often also called pixels; Figure 2.10:A). This data model often refers to so-called regular grids, in which each cell has the same, constant size – and we will focus on the regular grids in this book only. However, several other types of grids exist, including rotated, sheared, rectilinear, and curvilinear grids (see Chapter 1 of E. Pebesma and Bivand (2022) or Chapter 2 of Tennekes and Nowosad (2022)).

[Nowosad]

Thank you a lot! Especially, the grid->raster change is vital.