Add numerous checks to how the mapping.file is handled. Now checks
whether mapping.file can be opened before reading in data. Checks that
mapping.file is the proper .csv format. Performs a case-insensitive
check that mapping.file has a "filename" column. Throws a warning in the
event that mapping.file is missing some of the filenames that were read
in. Remove case sensitivity when mapping.file is a data frame, and again
checks whether a the object has a "filename" attribute.