Add "pediatric" in tumor TPM boxplot titles

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Currently, the boxplot titles are “Primary tumor and GTEx tissue bulk gene expression” and “Primary tumor tissue bulk gene expression”. Two example plots are listed at bottom.

Add "pediatric" in boxplot titles to show that the tumors are pediatric tumors. Proposed titles are list as following, and they are open to any change.

• “Pediatric primary tumor and GTEx tissue bulk gene expression”
• “Pediatric primary tumor tissue bulk gene expression”

cc @taylordm @afarrel @chinwallaa

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Closed with commit https://github.com/PediatricOpenTargets/OpenPedCan-api/commit/1b2493f160ff601595c04c196fb56360d16dc44a.