BUG: avoid overkill MuParser to perform simple affine function (avoid 4-digits MuParser issue on MacOSX)

This reverts commit 1a17cbca.

script must export LC_ALL=C to make sure the 'error' is echoed
correctly. Another thing is if there is a download error due to
proxy or firewall settings on the system. It should exit immediately
with a proper error message.

BUG: MANTIS-1112 (File not found if filename contains accents when using GUI) Replaced QString::toAscii() by QFile::encodeName() when converting filenames to std::string and used QFile::decodeName() when converting std::string filenames to QString ones.