Add explicit to single argument ctor
Add explicit to single argument ctor

wkentaro/chainer,chainer/chainer,wkentaro/chainer,chainer/chainer,chainer/chainer,keisuke-umezawa/chainer,okuta/chainer,niboshi/chainer,pfnet/chainer,niboshi/chainer,tkerola/chainer,okuta/chainer,keisuke-umezawa/chainer,hvy/chainer,okuta/chainer,keisuke-umezawa/chainer,hvy/chainer,niboshi/chainer,keisuke-umezawa/chainer,niboshi/chainer,wkentaro/chainer,chainer/chainer,hvy/chainer,okuta/chainer,wkentaro/chainer,hvy/chainer