Rework some abstract/generic examples to be practical ones

[pansapiens]

Common feedback we receive is that some participants can see 'why' they would use what they are learning, or how it might be applied. This is especially a problem for the early intro content and loops.

It would be worthwhile reworking some examples to have a practical looking application.

Eg, when reversing a string, use a DNA sequence, since we might want to do this as the first step of reverse complementing.

Eg, We could add some explanation to the jam_ratings challenge, that this is someone's ratings of different jam flavours. Then extend it to a list of dicts and calculate the average score for each jam type (which leads nicely into DataFrames)

[pansapiens]

Practical example that might be used: https://colab.research.google.com/drive/12T_RRzKBqClcY96oJHv077kO-yyOusNh#scrollTo=uDk1vLFIjMG9 - counting nucleotide frequencies in a DNA sequence.