introduce find function as a replacement for ls

Using ls with a wildcard, for example ls /dir1/dir2/*.txt has a length limitation. If the number of files is huge, the following error is expected: sh: /bin/ls: Argument list too long

I have encountered this error when producing 2D contour plot.

To solve the problem, I introduced a find fuction. This function will not produce the aforemntioned breaking error and is versatile for other uses.