heyoka 0.7.0

This new release of heyoka comes with several feature additions, a couple of fixes and support for the recently-released LLVM 12.

One important improvement is that the time coordinate in the adaptive integrators is now represented internally in double-length format, which greatly increases time accuracy.

As usual, the full changelog is available here:

https://bluescarni.github.io/heyoka/changelog.html