DEP: Raise FutureWarning when mode='full' is used in qr factorization.

The qr function currently returns a thin factorization when mode='full'.
That is misleading, consequently an extra mode 'thin' is added and 'full'
aliased to that. At some point is would be desirable to alias 'full' to
'big', so a FutureWarning is now raised when 'full' is used. Backward
compatibility may be maintained by letting the mode default as its default
value of 'thin' returns the same factorization as when the default value
was 'full'.
TST: Improve test coverage of qr factorization of complex matrices.

The previous tests only checked the case when the imaginary part was zero.
That would fail to properly check if the columns of q were orthonomal when
the Hermitean conjugate was used.