Allow CRC32c in place of MD5

The ASDF standard allows an optional MD5 hash for each block. While that has high sensitivity, it's a very slow function compared to a CRC (<1GB/s vs. >20 GB/s, slow enough to have a significant impact on large data sets), and a 32-bit CRC is adequate for many use cases. Would you be open to something like this? > ### Block header > ... > * `checksum` (16-byte string): An optional CRC32c checksum or MD5 hash of the used data in the block. See **flags**: If bit 2 is unset, then the value is the MD5. If bit 2 is set, then the value is the CRC32c followed by eight 0-bytes. The special value of all zeros indicates that no verification should be performed. > ### Flags > The following bit flags are understood in the `flags` field: > - `STREAMED` (0x1): ... > - `CRC32C` (0x2): The block header `checksum` is the Castagnoli CRC32 followed by eight 0-bytes. Thanks! --- I'm working in a task force to define the next version of the [FCS file format](https://en.wikipedia.org/wiki/Flow_Cytometry_Standard), and ASDF has a lot of attractive qualities. I might have a few more comments/questions coming over the following weeks.