version 0.5 ready to go

Merge pull request cta-observatory#73 from vuillaut/0.4.1

v0.4.1

making description readable for pypi

add astropy in requirements in README

fix version 0.3.0

v0.22

Merge pull request cta-observatory#30 from vuillaut/version

version 0.21

Merge pull request cta-observatory#25 from vuillaut/eff_area

refactoring of the cta irf

Update README.md