- Cleanup and docs to all Tensor.cs files
- Changed all uses of System.Convert to NumSharp.Utilities.Converts
- Added all missing types in dtypes.cs
- Renamed tensor.Data<T> to tensor.ToArray<T>, added obsolete message
- Renamed tensor.Data() to tensor.BufferToArray(), added obsolete message
- Made GraphKeys to use const string instead allocating strings at every use of GraphKeys.

Relative unit test in `OperationTest`.

Relative unit test in `OperationTest`.

Relative unit test in `OperationTest`.

Relative unit test in `OperationTest`.

Relative unit test in `OperationTest`.

Relative unit test in `OperationTest`.

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.

* removing unused namespaces

* implementing Tensor to scalar and scalar to tensor conversions

Change nest.flatten to generic.

Adding the `BatchMatMul` operation.
Expanding `BasicOperations` to test the new operation.