Seppl

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- added support to the `seppl.io.Splitter` class for keeping item/sample groups together via a `split_group`
regular expression
- backported helper methods for `seppl.io.Writer` classes for managing splitting

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- added `resume_from` parameter to `seppl.io.locate_files` method which allows to skip all
files preceding this glob

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- the `resolve_handler` and `split_args` methods now have the `partial` boolean
parameter which determines whether partial matches are accepted or not; off
by default as it can interfere with parameters from plugins

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- moved placeholder functionality from `seppl` to `seppl.placeholders`
- `load_user_defined_placeholders` now ignores lines that start with

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- added support for placeholders, which can be expanded via the `Session` object
- plugins supporting placeholders should import the `PlaceholderSupporter` indicator
mixin for automatically adding help on placeholders to the help screen; plugins
that support placeholders based on the current input should import the
`InputBasedPlaceholderSupporter` indicator mixin
- placeholder-supporting plugins can use the `placeholder_list` method in their argparse options
- the `load_user_defined_placeholders` method allows incorporating custom placeholders for directories

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- added alias support to the `ClassRegistry` class
- added method `is_alias(...)` and property `all_aliases` to the `Registry` and `ClassRegistry` classes
- extended the `enumerate_plugins` method to allow flagging of aliases (default: `*`)