Change names for ForwardPass

[alaksiej-stankievicx-dunnhumby]

As @jens-mueller-sociomantic and I were discussing: the names of enum ForwardPass and its values should be renamed to express, what happens.

One possible variant is:

enum ForwardPassMode
{
    OutputOnly = false,
    Objective    = true
}

(last filed can be also Gradient, but it worse describe what really happens)

Also we should not provide any default value for forward pass, that reader of code could see what exactly happens.