RStoolbox v0.4.0

Rewrite of RStoolbox, migration from raster to terra and sp to sf

New:

• RStoolbox moved on from the outdated sp and raster packages to sf and terra to ensure long term support of the tools.
• Thrown out unnecessary libraries

Fixes:

• rasterPCA(): Fixed a bug that caused the method and its unit tests to fail on Linux due to a corrupted covariance matrix calculated previously with terra::layerCor()
• superClass() unable to predict when there is NA in raster data (closes #102, reported by @bappa10085)