Describe GASB requirements for accounting for investment pools. Include $($a$)$ a discussion of when the use of an investment trust fund or a custodial fund is appropriate; $($b$)$ the portion of the investment pool to be included and excluded from fiduciary funds; $($c$)$ the basis at which investments are to be reported; $($d$)$ reporting of realized and unrealized gains and losses on investments; and $($e$)$ financial reporting $($i$.$e$.,$ financial statements$).$