Examples of polymers that may be used to make permeable cover 43 are cellulose esters and ethers such as cellulose acetate and cellulose butyrate and the other semipermeable film-forming compositions disclosed in U.S. Pat. No. 3,760,984 at col. 4, line 53 to col. 5, line 39 and in U.S. Pat. No. 3,995,631 at col. 7, line 40 to col. 8, line 15, the disclosures of which are incorporated herein by ref