* Add missing `operator >=`s

Also unit testing the new operators.

* Add missing `operator <=`s

Also unit testing all the operator cases.

* Add missing `operator >`s

Also unit testing all the operator cases.

* Add missing `operator <`s

Also unit testing the new operators.

Also unit testing the new operators.

* adding unit tests

* ignoring failing tests

* removing ignore from unit tests

Unit testing the operation too.

Exposing also the `IsFinite` one.
Unit testing both operations.

Meant to be a shortcut to directly set some basic configuration.

E.g.

```
var sessionOptions = new SessionOptions();
sessionOptions.SetConfig(new ConfigProto
{
   LogDevicePlacement = true,
   InterOpParallelismThreads = 4,
   GpuOptions = new GPUOptions
   {
      AllowGrowth = true,
      PerProcessGpuMemoryFraction = 0.12
   }
});
var session = tf.Session(sessionOptions);
```

Adding `AbsGrad`

To be aligned with Tensorflow, introducing also `Sign` operation
and its gradient.

- Concat
- LGamma
- Tanh
- Slice

* removing unused namespaces

* implementing Tensor to scalar and scalar to tensor conversions

Unit testing the gradient too.

Add missing `operator -`s