Adding to tolerance when comparing dx and dy

The comparison of dx and dy in [GRIDobj](https://github.com/TopoToolbox/topotoolboxr/blob/main/R/GRIDobj.R) (line 187) is too strict occasionally and require some tolerance (10^-9). Rather use [all.equal](https://stat.ethz.ch/R-manual/R-devel/library/base/html/all.equal.html) with a small tolerance. We had the the same issue in the MATLAB implementation.