elevation sorting and flow accumelation criteria for flow path to calculate Flowdir

[ecor]

I fixed some issues in flowDir function, introducing flow accumulation critaria for correction factor in converging flow apth and eleveation sorting when flow paths were tracked. This is current update. When I understand and look into rspatial code, I will submit further long documentation about flowDir function. Thank you best @ecor

[rhijmans]

Very good, thanks.

[kadyb]

From clang:

catchments.cpp:1166:7: warning: unused variable 'flowacci' [-Wunused-variable]
 1166 |   int flowacci=*(flowaccm+i);
      |       ^~~~~~~~
1 warning generated.