v2.7.5

• improved write 4 performance and introduced option to compress data
• rewrote xlsx export
• handle invalid bits using numpy masked arrays
• made convert_tables argument available for mdf 3.x
• added source in channel name when duplicated in file
• setup will not raise error in case cython is not installed
• replaced all print by warnings to be able to filter them
• fixed issues with numpy 1.14