Make handling of legacy input file name (without .mat extension) consistent

[mkoeppe]

"zbasis MyProject" loudly complains if file MyProject (without any extension) exists, even though it
is supposed to read MyProject.mat only. I then get the errors:

 INPUT ERROR: Badly formatted file MyProject 
 INPUT ERROR: Check the number of rows and columns. 
 INPUT ERROR: Check there are only integers. 

The other tools seem not to complain if a file with the name of the project already exists. 

(Reported by Bernard Berthomieu.)