Removed empty bytes at channel subchunk ends for issue #31

Update to reader.py for issue #35

Checks if the chunk data size does not match the expected, based on image axes size. Verifies that all unwanted bytes are zero-bytes and removes them. Two triggered warnings: one for unwanted bytes and one for bytes removed (more of an INFO though).

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.