Nexus search prototype

We want to enable apps to load the nexus metainfo on demand, although it is not a schema in the traditional sense.

Tasks:

• Apps can choose to load nexus definitions using glob syntax, e.g. 'nexus.NXiv_temp.*'
• Search bar and widgets should recommend definitions for compatible data. Since nexus application definitions can be very large, this requires virtualization when rendering e.g. lists.
• Search queries through the search bar should be possible
• The user-defined quantities menu should work for nexus as well

Issues:

• Most of the data lives in the nexus format seems to live inside repeatable sections (the metainfo section is flagged with repeats=True): both DATA and ENTRY are repeatable. The scatter plot widget and the row columns can currently only work with data that is unique for a single entry (=one scalar value per entry, as determined from the metainfo), and this is why none of the nexus data can be used there. The terms, min_max, and histogram aggregations work fine, as they just include all data. We have plans for going around this (e.g. in the results table, we can pick the min/max/mean value, even sorting is supported with these).
• Much of the interesting data (mean, min, max, etc. are stored inside special attributes (nx_data_mean, nx_data_var, etc). This data is not currently indexed or exposed in the search interface. By default, only the m_value field is stored and accessible, which I think contains the mean value..?

Todo:

• There are currently no nexus-specific filter menus (the menus on the left). It is possible to create one by hardcoding it in React, but the plan in the future is to be able to fully define these menus within the app definition (nomad.yaml).