support plotting survival curves for more than 2 groups

[jburos]

Came up in a recent conversation on slack; and, have personally wanted this feature for a while.

Some scenarios that came up in my experience:

• plot survival curves by a string variable or category, IE "pd-l1".
• user provides a function to map rows to groups, where the resulting groups number more than 2.
• user plots on a variable ("missense_snv_count") but provides more than 1 threshold value.

Copying convo from slack here for context:

@timodonnell :

I’d like to generate a survival curve for various levels of T cell infiltration

@tavinathanson:

how many curves in the plot? cohorts right now allows just 2 curves based on a binary feature or e.g. split by median value

[jburos]

Linking to related PR: #225. This PR implements the first two scenarios of the 3 listed above.