Nctoolkit

This release changes the default behaviour of `to_dataframe`. Bound type variables will now be ignored, as they are typically redundant once a dataset has been converted to a dataframe. For example a variable like `lon_bnds` will now be ignored. To retain the old behaviour use `to_dataframe(drop_bnds=False)`.

There are additional improvements to internals that do not modify package usage.

This is a minor release that makes improvements to `pub_plot`.

The `pub_plot` method will now display arrows bars on the colour bar when limits are supplied to make it clear values can exceed those shown in the colour bar.

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**Quieter warnings on Linux**. Improvements have been made to reduce the number of repetitive warnings on Linux.

The `pub_plot` method now lets you set the dpi in saved plots.

This is a minor release that fixes a bug that caused errors when `norm` was set using `pub_plot`. Everything else should work as before.

Minor bug fix release
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This is a minor release that improves how `pub_plot` handles some arguments to provide more consistent behaviour.

Adding the ability to create vertical level bounds
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This release adds a new dataset method `generate_level_bounds`, which will generate vertical bounds from the vertical levels. This is useful in case this info is not provided in the original data and they need to be estimated for vertical averaging etc.