Magpylib-material-response

* improved interface for chi input
* FE tests for anisotropic chi and negative chi
* Improve internals
* anisotropic susceptibilities are now allowed.
* Improve suszeptibility input possibilities:
- give susceptibility to parent collection
- if susceptiblity input is scalar, isotropic susceptibility is assumed, if it is a 3-vector it can be anisotropic
* Various tests included of interface and computation, isotropic and anisotropic tests confirm computaiton

* Fix null polarization for rotated objects ([7](https://github.com/magpylib/magpylib-material-response/pull/7))
* Fix docs not building ([6](https://github.com/magpylib/magpylib-material-response/pull/6))

* renaming xi->susceptibility ([5](https://github.com/magpylib/magpylib-material-response/pull/5))
* update to magpylib v5 ([4](https://github.com/magpylib/magpylib-material-response/pull/4))