added new tutorial for multichannel microscopy classification (#1108)

### Description
Added a new subfolder `./microscopy` and added a first tutorial for
6-channel image classification from multiplexed fluorescent microscopy.
The tutorial downloads a custom and newly create subset of the
[Rxrx1](https://www.rxrx.ai/rxrx1#the-data) dataset. This tutorial was
created in collaboration with Siddha Ganju (@sidgan) and Aastha
Jhunjhunwala (@aasthajh).

Co-authored-by: Seyed-Ahmad Ahmadi <mangotee@gmail.com>

Author: nvahmadi