Improve support for NRRD standard

- field names in lowercase
- support key:=value items
- support "content" field
- support .nhdr as a file extension
- support reading detached header in a different directory (fixes path issues)
- be picky about "dimension" and it matching other fields
- better splitting and try/catch fallback on parsing "space units" strings
NRRD: don't use mmap if need to bswap