columns in return values of `simulate_tree`

[sbfnk]

I think the names of the columns in simulate_tree and their documentation could be improved.

Especially:

• infectee_id tracks which of the index cases is the root of each infectee. Perhaps call it index_case_id?
• sim_id gives a unique id to each infectee that linked to a given index case (such that the combination of index_case and sim_id is unique. Should this perhaps not be called infectee_id? And be unique throughout the data frame (rather than unique for each index case)?

I'm also wondering if _id really needs to be added to the column names or whether they could just be index_case, infectee, infector.

So instead of currently something like

+   infectee_id sim_id infector_id generation time
1            1      1          NA          1    0
2            2      1          NA          1    0
3            3      1          NA          1    0
4            1      2           1          2    3
5            2      2           1          2    3
6            3      2           1          2    3
7            1      3           1          2    3
8            2      3           1          2    3
9            3      3           1          2    3

one would have something like

+   index_case infectee infector generation time
1            1       1       NA          1    0
2            2       2       NA          1    0
3            3       3       NA          1    0
4            1       4        1          2    3
5            2       5        2          2    3
6            3       6        3          2    3
7            1       7        1          2    3
8            2       8        2          2    3
9            3       9        3          2    3

[jamesmbaazam]

Closing as completed in #242.