Reduced the number of realloc calls zlib_mem_inflate(), while
hopefully not overallocating much either.

Forcibly set block->alloc field when decompressing from gzip or
bzip2. This caused issues in scramble, but doesn't seem to in samtools
due to no ability to set compression level higher. Fixing it here just
incase though.
Merge pull request #62, minor CRAM updates