New classes:
- CalculateWindow is a window to select what do you want to calculate
  and what tensor should be used in that calculation.
- OperationItems is a helper class contains allowable operations
  and tensors are needed for the operation.
- OperationRequirementWidget provides an ability to the user to choose
  tensor from the tensor store as for example 'Metric Tensor' or
  another required tensor.
Also, some other classes have changed.
Merge branch CalculateWindow with another commit.