Position reconstruction

[asogaard]

• Adds IdentityTask with configurable number of output nodes, that doesn't do anything and thus allows for "pass-through" of an affine transformation of the last hidden layer, i.e., without the application of any activation function or anything like that.
• Adds the option to use custom functions for the transform_* arguments to Task.
• Ignores validation of transform_target and transform_inference in favour of a warning in cases where these rely on indexing and strictly assume input tensors of dimensionality > 1.
• Adds config files for vertex position reconstruction, with and without custom transform functions
• Adds training example for vertex position reconstruction model, leveraging the above.

Closes #359

[RasmusOrsoe]

Thanks for this!