Add Lockable mixin

[psomhorst]

While working on tests for ContinuousData, I was fixing some locking mechanisms. Since these will probably be reused in the future, I decided to write a mixin instead.

This mixin has been implemented and tested with ContinuousData only.

[psomhorst]

On naming: the Lockable mixin class and the lock(), unlock(), islocked() and islockable() methods might suggest that the entire object is made lockable. This is not the case. Attributes in the object are lockable. We could rename this to:

• lock_attr()
• unlock_attr()
• attr_is_locked()
• attr_is_lockable()

but that becomes quite verbose.

[DaniBodor]

Could you comment on why this PR was closed (without merging)?

EDIT: I now see (or at least assume) that it has been superceded by #215.
Generally it's nice to have a short comment on why a PR/Issue was closed without being accepted. Even if it's just a reference to a new PR/issue.

[psomhorst]

@DaniBodor This was auto-closed by GitHub when I changed the branch name. I thought it would transfer the PR to the new branch name, but that turned out to not be the case. I was unable to reopen the commit and change the branch afterwards.

[DaniBodor]

Ah, i understand. I'm also surprised, it usually does transfer it in my experience