Consistency Check for Convergence and Magnification

[evevkovacs]

Since these are related, we should check that the shears, convergence and magnification are self consistent (within machine precision). The test is that magnification should be satisfy
'1/magnification = (1-kappa)2 - shear12 - shear2**2)'
(kappa = convergence)

[rmandelb]

@evevkovacs - totally minor/irrelevant point but for people new to GitHub, I wanted to point out that you can use quote marks to avoid your math getting reinterpreted as GitHub markdown:
1/magnification = [(1-kappa)**2 - shear_1**2 - shear_2**2]
And for reference to papers, magnification is often denoted mu, just as shear is often denoted gamma.

[yymao]

implemented in #66