helper function to crawl over the target location, determine if it is a file or folder, then locate all .inp files, and convert them to absolute paths
convert_to_filelist(target, filefilter = NULL, recursive = FALSE)
A vector of .inp file locaitons
a character vector where each element is a directory containing Mplus input files (.inp) to run OR a single .inp file to be run. Elements may be a full path, relative path, or a filename within the working directory.
An optional PCRE expression for filtering input files of interest
if TRUE, search for .inp files in subfolders of all elements of target