Fixed warnings on Windows (#407)

Now builds clean on MSVC: cl version 19.51.36260 for x64, from Visual
Studio 2026 Build Tools 18.10.3.

Major items:

- [[maybe_unused]] instead of gcc/clang specific pragmas
- Removed names variables that weren't used
- Cast + explicit uing64

src/JungfrauDataFile.cpp

@@ -221,7 +221,7 @@ JungfrauDataHeader JungfrauDataFile::read_header() {
         throw std::runtime_error(LOCATION + "Could not read header from file" +
                                  m_fp.error_msg());
     }
-    m_fp.seek(-header_size, SEEK_CUR);
+    m_fp.seek(-static_cast<ssize_t>(header_size), SEEK_CUR);
     return header;
 }