Index fund methods

New methods to compare ETFs with index (benchmarks) in AssetList:
- tracking difference
- tracking difference annualized
- tracking error
- index correlation
- index rolling correlation
- index beta

technical release

v0.87

Update README.md

technical release

v0.86

Update README.md

numpy<=1.19.3

v0.85

numpy<=1.19.3

verbose mode for EfficientFrontierReb

v0.84

v0.84

updated requirements

v0.83

- v0.83

stable version

v0.81

setup.py with correct classifiers

First public version

v0.8

setup.py with Pruduction Status