Upgrade science/hdf5-18 to 1.8.21.

Release notes at:
https://portal.hdfgroup.org/display/HDF5/Software+Changes+from+Release+to+Release+for+HDF5-1.8#SoftwareChangesfromReleasetoReleaseforHDF5-1.8-Release1.8.21ofMay2018versusRelease1.8.20

PR: 233071
Submitted by: wenheping2000 (at) hotmail.com (Original maintainer)