Pysigma

Fixed issue with installation of plugins in pipx virtual environments (and possibly others)

Added missing modifier class docstrings.

Additions

* Field existence check with `exists` value modifier.
* Added regular expression modifiers `i`/`ignorecase`, `m`/ `multiline`, `s`/`dotall`

Fixes

* Dropping all detection items of a detection caused error with `to_plain()` (104)
* Ability to set allowed backends from YAML processing pipeline (111)
* Merge of errors when `SigmaCollection` obejcts were merged (112)

* IPv6 CIDR support with *cidr* modifier.
* Fixed corner case of IPv4 CIDR support.
* Integrated pipeline to backend compatibility check into pipeline resolver.

Removed implicit storage of all keyword arguments from `Backend` class initialization into config object attribute to enable defined error behavior for unknown parameters instead of silently accepting them.

Silent plugin installation.