This is mostly a more graceful handling of bad concatenation requests.
Adds a check for stream counts to concatenation
Changes the check for empty mono tracks from "RMS peak" to "RMS level," which is the mean level of the selected track instead of the peak value (clicks and pops on an otherwise 'empty' track gave false positives)
Gives more detail on why a concatenation was not attempted
Simplifies the stats to check when comparing files for concat compatability
Placeholders for more complex concatenations (to be added later? I don't have time now and there doesn't seem to be an immediate need) where files differ in encoding/etc.
This is mostly a more graceful handling of bad concatenation requests.