Check for valid output directory for FieldGenerator methods that write files

[The9Cat]

Summary

Add checks using std::filesystem to check that the outdir parameter passed to FieldGenerator::file_lines, FiledGenerator::file_slices , and FieldGenerator::file_volumes is valid and throw a standard exception rather than terminate if the provided directory does not exist.

Tested by providing valid and invalid directories to FieldGenerator::file_volumes.

Comment

I also note that the standalone vtk volume writer appears to have transposed axes. Need to check this. The pyEXP.fields.volumes provides a 3-tensor that needs to be converted from column-major to row-major, but otherwise looks correct.

[michael-petersen]

Nice smooth change for the user. I hadn't run into this before, but looks helpful in general.