Improvements to ParameterSet

[alcrene]

Over time, I've accumulated a number of tweaks to my fork of parameters. (git diff)
These include

• Better support for NumPy objects (saving and comparisons);
• Deletion and value setting with dot notation;
• Allowing relative URLs when creating a ParameterSet from a file.

If you think any of these improvements are worthy of merging upstream, let me know and I'll create the relevant PRs.

[apdavison]

Hi, that would be great!