Fixed user-generated masks for training

[NilsBochow]

Hi,

is there a way to specify user-generated masks for the training instead of creating them on the fly? I know that @eliohead asked the same already (#119) , but I can't figure out if it is already implemented or if I have to do it on my own.

Thank you!

[NilsBochow]

I solved it. If someone is interested, just contact me.