Allow the python layer have attribute "phase"

[ZhouYzzz]

I noticed that the python layer cannot tell whether the phase is TRAIN or TEST. This change may bring convenience in some cases. e.g. to me.

class MyLayer(caffe.Layer):
    ... ...
    def forward(self, bottom, top):
        if (self.phase == caffe.TRAIN):
            # do sth.
        else:
            # do sth. else
    ... ...

[ajtulloch]

Looks fine to me. Would you be able to add a test that demonstrates reading out the phase with TRAIN/TEST values as well?

[shaibagon]

related to issue #3506 -- thanks for implementing this PR!

[ajtulloch]

Looks great to me. Can you squash the commits? @longjon , @shelhamer does this look good to you?

[longjon]

Scalars should be 0d; use top[0].reshape().

[longjon]

I never really got much into phase myself, but this is a clear improvement for the completeness of Python layers, so it seems worthwhile.

However, there are a few style issues, as noted. I'd be happy to merge once these are fixed (and squashed into the existing two commits).

[ZhouYzzz]

@longjon Thanks for review and I've fixed it. I'm glad this pr is helpful.

[longjon]

Looks very clean now, thanks!