Ignore default metadata

[alessandrofelder]

Addresses #37 and part of #41. Idea is to comment out the explanations to optional and required metadata parameters in the yaml files, so nwb_file writes pynwb defaults to file, instead of the explanations. Commented out explanations can be read using ruamel.yaml and displayed as tool tips in the GUI. Planned steps are:

• [ ] comment out optional explanations, write pynwb defaults
• [ ] comment out required explanations, write 'non-specified' as default
• [ ] switch to ruamel.yaml
• [ ] adapt GUI to read explanatory comments and display them as tool tips
• [ ] update signatures

tests currently fail due to changes in signature files. All functionality should have tests, too.

Some things yet to be decided:

• what do we do with actual comments like TODO that are not tool tips? Do we allow them in metadata.yaml?
• should unspecified numbers also be "not specified." strings or e.g. NaN?

[alessandrofelder]

Handled mostly in #45 (and signatures in #51 ). This was an attempt to pursue an ultimately a bit too complicated, and unnecessarily complicated, design approach.