add a check that filename passed to ND2Reader has extension .nd2

f-strings not supported by Python3.6-

add an exception InvalidFileType

add a test for the new behavior of ND2Reader constructor

Removed self.__class__ from super() call

Create CODE_OF_CONDUCT.md

Using Z slice coordinates from Z-data instead of Z-levels and show warning in case of mismatch

Changed isinstance to is.
Fixed condition triggering warning.

Fix to allow reading of ND2-files with missing frames.

Image events

include only valid loops in experiment metadata