pdftk now required by the Create Assignment Form script

[tvrusso]

Commit fb8a3d1 introduced a dependence on pdftk into IGT4SAR_fdf.py, which is callled by IGT4SAR_CreateICS204.py.

The script assumes that pdftk was installed from https://www.pdflabs.com/tools/pdftk-the-pdf-toolkit/, and that C:\MapSAR_Ex\Tools was given as the install directory to the Windows installer. It does not check for the existence of the pdtfk.exe file file in C:\MapSAR_Ex\Tools\PDFtk\bin directory before trying to execute it at line 48 using "subprocess.call(pdftk_cmd)".

It is easy to fix, just install PDFtk as described on the linked web site. But the consequence of not having it is a fairly cryptic series of errors from Python reporting that a file was not found, without actually saying what file it was (it was, of course, pdftk.exe).

A simple check for the existence of the binary and a clear error message explaining how to fix the problem would be helpful.