Rt1

- fix error in index-order of res_df

- add version-number to dump-files
- avoid concatenation in jac-calculation
- faster way of getting _groupindex

- `intermediate_residuals()` added to `rtplots`
- allows analyzing residual-development during fit-procedure with 2D or 3D plots
- fixed issues with unsorted (manual) parameter-groupings
- add possibility to use the `dataset-index` or the `group-index` in `printsig0analysis`
- add public method `calc_model()` as wrapper of `_calc_model` that directly returns a `pandas.DataFrame`
- get zenodo DOI and automatically deploy releases to pypi.org

First release of the RT1 module, consisting of:

- `rt1.rt1`: basic calculation routines for symbolic and numerical evaluation of generic models
- `rt1.surface` and `rt1.volume`: a set of flexible surface- and volume scattering phase functions
- `rt1.rtfits`: interface for (parallelized) nonlinear least-squares fitting
- `rt1.rtplots`: several nice plot-functions and interactive widgets to analyze results
- `rt1.rtparse`: a configparser to specify processing-routines via `.ini` files
- `rt1.processing_config` : some useful default functions for processing

this is (still) a beta-release, so use it with care!

- major updates of rtfits
- parallel processing
- configparser
- use properties to avoid storing duplicated data
- use caching to speed up fits

- additional useful features added

this is a beta-release, so use it with care!

- the fit.result list has been removed and replaced with individual class-attributes
- several plotfunctions added
- setter- functions for parameters are removed (from now on, no axis is added when setting a value)