Update pipeline class to handle partitioned arrow files

[ddobie]

Planned upgrades to the VAST pipeline will mean that the measurements.arrow file becomes a partitioned file (i.e. measurements.arrow will become a directory containing several parquets).

Vaex can handle this, but the syntax is slightly different (you need to point at the individual parquets rather than the directory that contains them). Hence, vast-tools needs to be updated to handle this option too.