RCP checker refactor: load one benchmark

[maanug-nv]

• Split check_directory() from RCP_Checker class
• RCP_Checker created for only 1 benchmark, only reads RCPs and maintains state for 1 benchmark
• RCP_Checker state and stats computed (compute_rcp_stats()) at initialization
• check_directory() creates a checker with benchmark read from get_submission_epochs()

To check a results directory, now only need to call check_directory()

Depends #289 , #293

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[pgmpablo157321]

@maanug-nv can you resolve the conflicts so that we can merge this PR?

[maanug-nv]

Rebased on latest master, no conflicts now 👍