v0.7.0

ER2 0.7.0

The first release of ER2 on PyPI, and the one that makes ER2 a specified language:
docs/LANGUAGE.md now defines it,
rule by rule, and every rule is checked by a test. It brings together three milestones that
had built up since 0.4.2: algebra, series and the language specification.

pip install er2              # core
pip install "er2[jupyter]"   # plus the Jupyter kernel
pip install "er2[oeis]"      # plus OEIS access