Postpetition income taxes on sale of farm property by Chapter 12 debtor are not “incurred by the estate,” are not administrative expenses under § 503(b) and are not entitled to priority; arguably in dicta, same result abides in Chapter 13: postpetition taxes are incurred by debtor and are not collectible unless government exercises option under § 1305 to seek collection within the Chapter 13 case. “In Chapter 12 and 13 cases, . . . there is no separately taxable estate. The debtor—not the trustee—is generally liable for taxes and files the only tax return. . . . Section 1399 of the IRC, much like § 346(d) in the Bankruptcy Code, clarified that the estate is not separately taxable in Chapter 13 (and now Chapter 12) cases. . . . A provision in Chapter 13 confirms that postpetition income taxes fall outside § 503(b). Section 1305(a)(1) . . . gives holders of postpetition claims the option of collecting postpetition taxes within the bankruptcy case—an option that the Government would never need to invoke if postpetition tax liabilities were already collectible inside the bankruptcy. . . . [P]ostpetition income taxes are not automatically collectible in a Chapter 13 plan and, a fortiori, are not administrative expenses under § 503(b).”