valarray context must be system::default_context()

[jszuppe]

valarray needs some fixing. Currently, it's possible to create a valarray object with buffer in some custom context that is not the context of system::default_queue() which is used in every method... And that breaks the code.

Since valarray is not a popular feature, I think we should make it work only in system::default_context(). Otherwise, we have two other options:

1. force (almost) every operator and method to create a queue based on context of underlying buffer (m_buffer) and work synchronously,
2. keep command_queue object in valarray.