Seppl

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- `check_compatibility` method now also checks whether generated class is
subclass of accepted classes, to allow for broader `accepts()` methods
- `gcd` method now creates a copy of the integer ratio list before processing it

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- added basic support for meta-data: MetaDataHandler, get_metadata, add_metadata
- added support for splitting sequences using supplied (int) split ratios
- added session support: Session, SessionHandler
- added I/O super classes: Reader, Writer, StreamWriter, BatchWriter, Filter, MultiFilter
- added support for executing I/O pipelines: Reader, [Filter...], [Writer]

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- the `DEFAULT` placeholder in the environment variable listing the modules now
gets expanded to the default modules, making it easier to specify modules
in derived projects
- added `excluded_modules` and `excluded_env_modules` to `Registry` class
initializer to allow user to specify modules (explicit list or list from env
variable) to be excluded from being registered; useful when outputting
help for derived modules that shouldn't output all the base plugins as well.

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- the registry now inspects modules when environment modules are present even when
it already found plugins (eg default ones)

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- the registry now inspects modules when custom modules were supplied even when
it already found plugins (eg default ones)

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- suppressing help output for unknown args now