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matthew-brett 
Merge pull request #391 from MarcCote/streamlines_api

MRG: Streamlines API

Provides a general API to manage different streamlines file format (the code can all be found in `nibabel.streamlines.`). This is related to this somewhat outdated [BIAP5](https://github.com/nipy/nibabel/wiki/BIAP5).

This new streamline API is divided in two main parts:

1. Representation of streamlines in memory (`Tractogram` and `ArraySequence` classes)
2. Loading and saving streamlines from/to different file format (`TractogramFile` class)

*We use the term 'Tractogram' to refer to a set of streamlines, usually the output of a tractography algorithm.*

## ArraySequence

The `ArraySequence` class needs to be explained first as it is at the heart of the streamlines representation. I guess it could be seen as a sort of jagged/ragged array to store a list of 2D arrays that have different length but a same number of dimensions (e.g. a list of list of points). For efficiency, we store a list of streamlines contiguously in memory using what we call a `ArraySequence`. This was needed as @Garyfallidis and I realized that a list of numpy arrays has too much overhead in term of memory when handling >1M streamlines (loading a tractogram of 2.5Gb could take up to 5Gb in RAM!).

An `ArraySequence` behaves exactly like a `list` of `2D ndarray`. That is, it supports appending, extending, indexing (supports slicing too), iterating, etc. as a `list` object. For example, indexing a `ArraySequence` object will return a 2D `ndarray`, as one would expect, representing the 3D points of the selected streamline. Note that, while `ArraySequence` does not support deletion (i.e. no `del` nor `remove`), one can efficiently use slicing to achieve the same goal.

An `ArraySequence` object can be instantiated using any iterable yielding `2D array`:

```
import numpy as np
import nibabel as nib
data = [np.random.rand(10, 3), np.random.rand(3, 3)]
seq = nib.streamlines.ArraySequence... (continued)

20267 of 21090 relevant lines covered (96.1%)

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Uncovered Existing Lines

Lines Coverage ∆ File
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Jobs
ID Job ID Ran Files Coverage
5 979.5 (COVERAGE=1) 0
96.1
 Travis Job 979.5
Source Files on build 979
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