Load weights during testing, apply them to the PDF model

Created by: scarlehoff

In order to reduce a bit the entropy of the tests I've added a weights file so that all runcards start at the same point.

One feature I've adding while doing this is to apply the weights to the PDF which brings the advantage of being able to train the PDF with runcard A and then continue the training in runcard B (the architecture needs to be the same, obv). Added a point about that to the docs.

Note that by doing this we "lose" information about whether the initializer did start at the same point but that should be tested separately in any case because we are only testing one single initializer with this runcard.

Note 2: a lot of files in layers have changed but most of them are a rename from meta_call to call, which is a bit of a test-commit because I am not sure if the mac version will like that. If it does it brings n3fit a bit closer to the standard tensorflow naming conventions.