PyramidLevelLayer for spatial pyramid pooling [under development, don't merge]

[ronghanghu]

Note: DO NOT use the code now, for it is incomplete and under development.

This is a PyramidLevelLayer implementation, which can be used to build spatial pyramids, especially designed for #560 (Spatial Pyramid Pooling).

The pooling region is calculated in the following way:

A pooling region starts with hstart (include) and ends with hend (exclude):
hstart = floor(ph * bin_size_h_)
hend = ceil((ph + 1) * bin_size_h_)
where bin_size_h_ = float(bottom[0]->height()) / bin_num_h_ is float-point bin length
and similar for wstart and wend

I will add tests and update.

[ronghanghu]

@kloudkl this is my implementation for PyramidLevelLayer (still under development, haven't been tested yet). Once tested, it can be used for spatial pyramid pooling.

[kloudkl]

The first version of #560 looked exactly like what you did here. It copied the related layers instead of directly re-using them. Please refer to @shelhamer's and @Yangqing's comments on the original implementation of #560.

The latter was deleted but still sent to my email. The words are pasted here FYI.
I agree with @shelhamer ... Spatial pooling should most likely be carried out by normal pooling layers followed by a concatenation layer (we can write one if it does not exist). The current solution provides too much duplicated code - essentially another pooling function.

Please help review a simpler solution in a189891 "Pooling layer allows float heights and widths for kernels and strides". It builds upon your rectangular pooling layer. Thanks!