reporting: change default aggregation

[leondz]

Reporting tweaks:

• change default module-level aggregation to lower quartile
• note aggregation method in tooltip
• fix group template name to match passed variable
• remove extraneous text in group template
• capitalise report heading

[erickgalinkin]

I've not tested it but assume you did. I like the move here.

[leondz]

I've not tested it but assume you did. I like the move here.

tested with

• show_top_group_score = true/false, checking for present/absent group-level figure
• group_aggregation_function lower_quartile/mean_minus_sd/proportion_passing having just one probe (-p tap), looking for crashes and correct report content
• calling statistics.*() (for relevant members of *) to check behaviour with lists of length 1