Check input format; revise download script

* It is now okay to give any name to the input matrix. The library won't
  assume its name to be 'A'.
* Makefile now invokes get_matio.sh just before building spmat.o. Build
  works whether the user types 'make' or 'make tester'.
* If the tarball for MAT I/O library failed to download, get_matio.sh
  will remove the incomplete copy and download another copy anew.