Improve precision of v_from_i for large arguments

[cwhanse]

Number of iterations of Newton method should increase with the order of
the argument, to maintain precision. Not expected to have a significant
effect on most calculated IV curves.

[wholmgren]

Looks like the tests are failing for two reasons: 1. order needs to be a scalar, and 2. order needs to be an integer. Something like this might work

order = np.floor(np.max(np.log10(w))).astype('int')

Finally, please add a note and your name to the docs/sphinx/source/whatsnew/v0.4.2.txt file too.

[cwhanse]

Closing this to submit a clean set of files