Add ts.__str__ method

Add method to stringify timestream instance, and test for it
Changes to be committed:
	modified:   tests/helpers.py
	modified:   tests/test_classes.py
	modified:   timestream/__init__.py
Add a getFeature to ImagePotHandler

* timestream/manipulate/pot.py (getFeature): New method that fetches the feature
by feature name
_pixels orthogonal from _path, _datetime and _timestream

TimeStreamImage was trying to keep _pixels, _path, _datetime and _timestream
consistent. However, _pixels is orthogonal from the other three; we need to be
able to set _pixels without changing _path and viseversa. This commits removes
the checks to _pixels.
* timestream/__init__.py (TimeStreamImage.read): Change method to force a
refresh in _pixels. Using this method is a way to replace a _pixels by
forcing a reading of _path.
(TimeStreamImage.pixels): Removed some logic and put it in the read method. We
call the read method from here.
(_pixel): Remove checks of consistency between _pixels and _path
Use the same TimeStreamImage instance to write to stream

* timestream/manipulate/pipecomponents.py (__call__): Use the incoming
TimeStreamImage to output to timestream.