Separate activity/route data pull & data prep/visualization into different scripts

[francine-stephens]

Currently, the template strava_api.R script contains both the data ingestion as well as cleaning and visualization. Let's create one script that just holds the data ingestion for:

1. athlete data
2. activity data
3. routes data

Then, create separate scripts for data cleaning and processing specific to the visualization being created, e.g.:

1. heat mapping,
2. elevation profiles
3. Activity summary metrics.