High-level documentation for using MP-CNN and VDPWI (forward inference)

[lintool]

@tuzhucheng Can you check in a MP-CNN model in Caster-models/ and write up instructions on how to actually use it? I should be able to open a Python shell, copy-and-paste a few commands, and be able to run forward inference. This way I can start playing with the code...

@daemon can then follow up with the same for VDPWI.

[tuzhucheng]

The MP-CNN portion is done in #111 and improved in #119.

@daemon I think for VDPWI you just need to store the models in Castor-models and put in the instructions for running pre-trained models similar to MP-CNN.

[lintool]

@daemon not's not store a not-so-good model :)
Let's store once we get up to replication numbers at least?