Segmentation network

Changelog ...

Disable dropout @IvanKuchin (#40)
Change DSC gamma 0.001 -> 1.0 (suppose to improve generalization) @IvanKuchin (#35)
wip: tile dataset @IvanKuchin (#29)
Pancreas min/max HU calculation @IvanKuchin (#26)
Model output shape replaced from array to a single tensor @IvanKuchin (#25)
Change pacreas HU from [-512, 1024] -> [-310, 390] @IvanKuchin (#23)
Loss function changed as described: https://arxiv.org/pdf/1803.05431v2 @IvanKuchin (#20)
syncing model builds in tests with config @IvanKuchin (#19)
Turn off debugging output @IvanKuchin (#18)
disable terminateonNaN @IvanKuchin (#16)
save checkpoint on each epoch @IvanKuchin (#15)
performance improvements @IvanKuchin (#10)

🚀 Features

Feature: add regularization @IvanKuchin (#38)
Feature: add metric IoU @IvanKuchin (#36)
Feature: add loss function factory @IvanKuchin (#33)
Feature: different ways of calculating slice thickness @IvanKuchin (#32)
Feature: different ways of calculating slice thickness @IvanKuchin (#31)
Feature:Added factory of tile/no-tile prep DS @IvanKuchin (#30)
Feature: tiling training dataset @IvanKuchin (#27)
Feature: added GRADIENT_ACCUMULATION_STEPS @IvanKuchin (#24)
Feature: significant architecture changes @IvanKuchin (#22)
Feature: added DICE loss @IvanKuchin (#21)
Feature: added pancreas histogram @IvanKuchin (#17)
Feature: rescale dataset to the target resolution and different zoom levels @IvanKuchin (#12)
Enhancement: inroduce config parameter TRAIN_PASSES_PER_VALIDATION @IvanKuchin (#9)

turn off debugging @IvanKuchin (#39)
Bugfix: incorrect Z-axis orientation @IvanKuchin (#34)
Bugfix: random number generator must be [low, high) @IvanKuchin (#28)
Bugfix: incorrect import statement @IvanKuchin (#14)
turn-off debugging @IvanKuchin (#13)
Regression commit f009de5: consider only .nrrd and .dcim files @IvanKuchin (#7)