Extract the .zip files in memory

[mibe]

Don't extract and delete the .hgt.zip files, but when loading these files extract them in memory. This will save some space on disk.

[hgy59]

IMHO this is not required.

• diskspace is cheap
• zip files come with different extensions (see my comment in Add new function to source files locally #15)
• the cache is based on .hgt files and there is no need to change this