tuttleHost: check values that the plugin has returned to the IsIdentity action

The plugin could declare itself as an identity node. In that case, it
returns the name of the input clip and the time to use.
Throw an error:
* if this clip name is not a valid input clip
* if this input time is not available
The input time is not available means that the identity time asked by
the plugin is not contained in the frame range declared by the
framesNeeded action.

Issue #290
tuttleHost: add todo to properly disable branches using isIdentity
tuttleHost: getFullName is common to all clip types
tuttleHost: add minor utility functions
tuttleHost: avoid random values

But should never be used uninitialized...
tuttleHost: minor formatting fixes
tuttleHost: bug fix to use multiple times of the same clip

A plugin could use multiple times of the same input clip using the
getFramesNeeded action.

Fix #291
tuttleHost: timeDomain action before setup