ENH: extra volume preparation

Adding the ability to remove poor gates based on the normalized coherent
power and cross correlation ratio fields, assuming prep = 1. This is
added (quite easily) using an additional kwarg called extra_prep. This
should further help the 4DD algorithm by removing noise from the
receiver and second-trip echoes. I decided to add this feature as I
noticed the lowdbz and highdbz thresholds for the reflectivity field
were not adequately removing enough bad gates, especially those
representing second-trip echoes.
DOC: extra prep docs

Adding documentation necessary for the extra volume preparation inputs.
I am also adding in a check to make sure that the radar has the
necessary fields to perform the extra volume preparation, assuming
extra_prep is True.
MAINT: 4DD extra prep does not require copy of radar
All is good!

I tested @jjhelmus' pr_175 branch, which sought to remove the
unecessary copying of all radar fields from my original PR to improve
velocity correction using the 4DD algorithm. After testing and
correcting some minor bugs (just typos), it looks like this PR is ready
to merge into Py-ART's master!
Merge branch 'jjhelmus-pr_175' into fourdd-prep
Merge pull request #175 from kirknorth/fourdd-prep

ENH: 4DD extra volume prep