Add Colab support

[giswqs]

Colab now supports ipyleaflet (googlecolab/colabtools#498 (comment)). Users need to run the following two lines at the beginning of a Colab notebook in order to display the map. Adding these two lines to __init__.py will make it easier to use ipyleaflet on Colab.

from google.colab import output
output.enable_custom_widget_manager()

Reference: #864

I already updated geemap and leafmap to use the ipyleaflet plotting backend on Colab. Check out the Colab example: https://gishub.org/leafmap_colab

[martinRenou]

I moved your commit to this branch and opened a new PR #871

Please open PRs from other branches than master :) It makes things easier, I don't want to mess with your master branch