emptyness passthrough

the non-writing functions now return empty-like values when empty filename or other parameters are passed in. This seems to be a better match to expected behavior of other Matlab functions: empty in, empty out.

• {h5,nc}exist
• {h5,nc}size
• {h5,nc}variables

Separately, we moved CI to Azure since Travis-CI now has quotas, even for open-source projects.