Changes from Mercurial (hg https://bitbucket.org/isa-afp/afp-devel/ default)
Summary
- merge from afp-2019
- new entry Linear_Inequalities
- Nullstellensatz website
- new entry Nullstellensatz
- new entry: Groebner_Macaulay by Alexander Maletzky
- New entry IMP2_Binary_Heap
- update usage instructions to 2019
- add 2019 releases for existing entries
- update release dates for 2019