#33 Move to TreeCorr for correlation function tests

[msimet]

This is a PR to merge code that uses the TreeCorr package rather than corr2. A lot of this is inside baseball for the workings of the CorrelationFunctionSysTest, but I will highlight in particular:

• Most of the code is now gone from corr2_utils.py (now treecorr_utils.py). You have no idea how excited I am about this.
• The call signature for the CorrelationFunctionSysTests has changed: now it is sys_test(data, [data2, random, random2,] config=config_dict, **kwargs) instead of the old sys_test(config_dict, data, [data2, random, random2], **kwargs). The new form is more in line with the call signatures of the other SysTests, and I think it's clearer too.
• Various places that "corr2" has been changed to "TreeCorr."
• As long as I was changing imports, I went through and tidied up extraneous import calls in all files in the stile directory (all of which were, I'm sure, my fault in the first place).
• Added a fieldNames dict to stile_utils.py, so we have an obvious in-Python place to look up what the fields should be called. (Possibly we should do this for object_types too?)
• General PEP8ifying (spaces after commas, no trailing spaces on lines, etc)
• Added a CHANGELOG.

Unfortunately, due to both rewriting a lot of the file & changing its name, the GitHub interface thinks stile/treecorr_utils.py and stile/test_treecorr_utils.py are entirely new files. Hopefully it's not too terrible to read, though.

[HironaoMiyatake]

I got the following error and found that this is due to recent change on API in TreeCorr (G2Correlation -> GGCorrelation). I think you assume we use v3.0, but it might be better to adjust that to v3.1. http://rmjarvis.github.io/TreeCorr/html/changes.html#api-changes Actually this v3.1 was released after your PR, so my slow review should be blamed!

[miyatake@tiger3 Stile]$ python examples/example_run.py 
ERROR: RuntimeError: module compiled against API version 9 but this version of numpy is 6 [astropy.io.fits.hdu.compressed]
RuntimeError: module compiled against API version 9 but this version of numpy is 6
ERROR: AttributeError: 'module' object has no attribute 'G2Correlation' [unknown]
Traceback (most recent call last):
  File "examples/example_run.py", line 3, in <module>
    import stile
  File "/tigress/HSC/users/miyatake/Stile/stile/__init__.py", line 8, in <module>
    from .sys_tests import (GalaxyShearSysTest, BrightStarShearSysTest, StarXGalaxyShearSysTest,
  File "/tigress/HSC/users/miyatake/Stile/stile/sys_tests.py", line 98, in <module>
    treecorr_func_dict = {'g2': treecorr.G2Correlation,
AttributeError: 'module' object has no attribute 'G2Correlation'

[msimet]

Okay, I think this is back up and running now. The bug Hironao found has been fixed, and I realized that some different correlation function types weren't fully supported, so those have been added as well.

If you're interested in doing a code review on this branch...please do! Thanks!

[msimet]

OK, I think I've addressed everybody's comments so far...I'll give it a few more days to let people look over the changes, and merge perhaps Tuesday night unless further changes are needed.