NIRCam class doesn't auto-select channel based on detector attribute

[mperrin]

The docs suggest it does, but actually this doesn't work like it should:

In [3]:nc = webbpsf.NIRCam()
In [4]: nc.detector = 'NRCA5'
In [5]: nc.pixelscale
Out[5]: 0.0311          # UNEXPECTED - this is still the SW pixel scale. :-(

Discovered in the course of checking my suggested fix options for #216