Catlearn

- Added module for site featurization and GA feature selection.
- Fixed ML-NEB compatibility issue with FHI-AIMS ase calculator.
- Compatibility updated for ASE 3.19.0
- Compatibility updated for Pandas 0.24.0
- Compatibility updated for Scikit-learn 0.22.0
- Dropped support for python 2.
- Dropped support for python 3.5
- Added testing for python 3.7 and 3.8

- Fixed compatibility issue with MLNEB and [GPAW](https://wiki.fysik.dtu.dk/gpaw/index.html)
- Various bugfixes

- Added ML-MIN algorithm for energy minimization.
- Added ML-NEB algorithm for transition state search.
- Changed input format for kernels in the GP.

- Restructure of fingerprint module
- Pandas DataFrame getter in FeatureGenerator
- CatMAP API using ASE database.
- New active learning module.
- Small fixes in adsorbate fingerprinter.

- Major modifications to adsorbates fingerprinter
- Bag of site neighbor coordinations numbers implemented.
- Bag of connections implemented for adsorbate systems.
- General bag of connections implemented.
- Data cleaning function now return a dictionary with 'index' of clean features.
- New clean function to discard features with excessive skewness.
- New adsorbate-chalcogenide fingerprint generator.
- Enhancements to automatic identification of adsorbate, site.
- Generalized coordination number for site.
- Formal charges utility.
- New sum electronegativity over bonds fingerprinter.

- `ConvolutedFingerprintGenerator` added for bulk and molecules.
- Dropped support for Python3.4 as it appears to start causing problems.