Teaching Time versus Teaching Times?

While working on our deep-learning-intro lesson, the data scientists in us bumped our heads onto the question how to log individual teaching times per teaching module in the current lesson setup. The quick hack we came up with was to add a comment to the `teaching` field in the jekyll front matter.

For example, something like this:
```
---
title: "Classification by a Neural Network using Keras"
teaching: 30 # psteinb:50
exercises: 30 # psteinb:10
questions:
- "What is a neural network?"
```

For me as an instructor, a distribution of times helps tremendously to establish a trustworthy reference.

See also:
https://github.com/carpentries-incubator/deep-learning-intro/issues/124