Support for YAML 1.2

As discussed over at https://github.com/gammapy/gammapy/pull/2218#discussion_r301211357, @bsipocz asked me to raise this issue here.

the pyyaml library only supports the YAML 1.1 standard from 2005, while only YAML 1.2 added support for scientific notation floating point values.

Summary of differences here: https://yaml.readthedocs.io/en/latest/pyyaml.html

This is why many packages, e.g. conda itself, switch to ruamel.yaml: https://yaml.readthedocs.io/en/latest/index.html

[taldcroft]

One thing about ruamel_yaml that should be considered is that it appears to be in the midst of a major API change with version 0.15+: https://yaml.readthedocs.io/en/latest/api.html. The docs say that the API is still being fleshed out and production could should stick with <0.15.

That API change might be a good thing in the end, but it seems that right now it might just introduce a lot of churn and trouble for astropy to first transition to the <0.15 API and then we have the mess of possibly supporting <0.15 and 0.15+ ruamel_yaml.

tl;dr - I would propose to wait for ruamel_yaml 1.0 and a stable API before seriously considering a transition for astropy.

[pllim]

There is an ASDF PR but it is not merged yet; xref spacetelescope/asdf#677