seekg calls in ls-mat5.cc

[John W. Eaton]

There are a number of places in ls-mat5.cc where we have things like

   is.seekg (tmp_pos + static_cast<std::streamoff> (PAD (len)));

This seems risky to me because I don't think there is any guarantee
that a cast from an integer type to a std::streamoff type is
guaranteed to work (std::streamoff might not be implemented with a
simple integer data type).

Is it really necessary to save and restore the file position when
reading MAT files or is there another way that we could implement
these functions to avoid all the calls to seekg?

jwe