Add support for standardized and well-known input data format (stop using `matrix` everywhere!)

[AlePalu]

fdaPDE must be able to support many different data format as input, well established in the R language and scientific community. Is quiet annoying to input data in low-level matrix or array data formats. This is true for statistical data, but also for the domain geometry (this is a critical aspect which must be fixed sooner or later).

This issue wants to keep a list of all data format we should (or wish) support to enhance the fdaPDE usage. Each iter reports the data format, its supporting R package and possibly some specific documentation:

• netCDF: widely used format for exchanging or distributing climate data, and has also been adopted in other fields, particularly in bioinformatics, and in other disciplines where large multidimensional arrays of data are generated (https://pjbartlein.github.io/REarthSysSci/netCDF.html). Supporting packages: ncdf4, raster.
• raster: a raster is a spatial (geographic) data structure that divides a region into rectangles called “cells” (or “pixels”) that can store one or more values for each of these cells. (https://cran.r-project.org/web/packages/raster/raster.pdf). Supporting packages: raster, terra (https://github.com/rspatial/terra)
• data.frame: data frames are one of the basic dataset structures for the R language. Also support for modern packages like tibble would be nice (https://tibble.tidyverse.org/).