Cannot subset NetCDF for an area with multiple variables

[JustinElms]

Describe the bug
Users get an 'Internal Service Error' when sub-setting a NetCDF file with multiple variables. This occurs for any combination of variables, all types of areas (drawn, entered, uploaded, and predefined), and all of the datasets I tested with (GIOPS 1, RIOPS 7, CCG 5, CIOPS 9, and CMEMS 16). The 'Compress as *.zip' option has no effect on outcome.

To Reproduce
Select any dataset then produce an area plot using any method. Once the plot has loaded select multiple variables in the Subset panel and click save. This results in an 'Internal Server Error' page.

Expected behavior
Either a NetCDF file or a zip file containing one should be downloaded after completing the above steps.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Ubuntu 20.04 / Windows 10
• Browser: Chrome / Firefox / Edge
• Version: 92.0.4515.159 / 91.0.2 / 90.0.818.41

[JustinElms]

Some of this behaviour appears to be related to Issue #919, while other instances are due to conflicting unnamed dimensions which get labelled 'dims_0', ''dims_1', 'dims_2', ... .