Nctoolkit

This is a minor release that enhances the `bottom` method.

You can now specify if you want `bottom` to extract the bottom level in the dataset, or if you want to extract the bottom-most level with non-missing values.

A couple of additional improvements have been made to nctoolkit internals.

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.