Ensure only netCDF or PP ancillary files are passed to Iris

Migrated Jira Issue CDDSO-17
Jira creation date 21/04/2022 15:23
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When processing PP //model output files//, the warning `Unable to interpret field 0. Unsupported header release number: -333512704. Skipping the remainder of the file.` is issued. This is due to the fact that the `ancil_files` list contains both netCDF and PP files (this warning occurs when the PP loader attempts to load netCDF files). It should be possible to pass only PP or netCDF ancillary files to Iris, depending on the extension of the //model output files//.
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Start by tracing where the ancil_files list is assembled and how the model output file extension is identified. Confirm that Iris receives only ancillary files matching the model output format, and verify that the unsupported-header warning no longer occurs when processing PP output.

Written by the indexing model from the issue text.