Change boxplot titles to describe primary/relapse of included tumor samples

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Change boxplot titles to describe primary/relapse of included tumor samples, according to @taylordm's suggestion.

Currently, all titles use "Pediatric tumor" to describe tumor samples.

Following are the proposed changes of the "Pediatric tumor" to describe primary/relapse of included tumor samples.

• "Pediatric primary and relapse tumor" for boxplots with both primary and relapse tumor in the same box or different boxes.
• "Pediatric primary only tumor" for boxplots with only primary tumors.
• "Pediatric relapse only tumor" for boxplots with only relapse tumors.

cc @chinwallaa @afarrel