Add loss, lrDecay and lrCallback class

add classes to modify the default training methods. Think about module name (or even split in different models?)

• losses: Create your personal loss function
• lrDecay: Setup up a decaying learning rate
• lrCallback: Setup up callback

added enhancement label

changed time estimate to 2h

added 27m of time spent at 2019-11-11

marked the checklist item losses: Create your personal loss function as completed

mentioned in commit 38849c46

mentioned in commit 602ecea1

marked the checklist item lrDecay: Setup up a decaying learning rate as completed

marked the checklist item lrCallback: Setup up callback as completed

lrCallback is not used anymore and will be deleted

added 2h 8m of time spent at 2019-11-12

assigned to @leufen1

closed via commit 602ecea1

mentioned in commit 7e9cd7eb

closed via merge request !9 (merged)