HDiv boundary recovery not working for vector function spaces

[tommbendall]

We saw some odd behaviour with lop-sided bubbles (see #688). I tracked this down to the HDiv boundary recovery, which involves:

• separating the vector field into scalar components
• performing the boundary recovery on the scalar components
• recombining the scalar components to obtain a vector

This process of separating and recombining components uses interpolation -- and this seems to be the issue:

• if I comment out the boundary recovery, the bubble is still lop-sided
• if I use a projection instead of interpolation, the bubble looks good

This needs understanding and fixing! #688 got away from this by turning off the HDiv boundary recovery