Benchmarking database construction

[wtbarnes]

Building the HDF5 database can take an annoyingly long time. This is fine when you only have to do it every once in a while, but is increasingly becoming an issue when running the tests as our list of files in the test database gets larger.

We should benchmark the construction of the database and find where the really slow bits are. There are undoubtedly several bottlenecks that are causing things to be really slow. This would make running the test suite faster as well as reduce the CI time.