Added parameter float_precision to CSV parser

closes #8002.
> @mdickinson can you point to some examples that this would work differntly?

Hmm; it's been a while, and I don't have examples readily to hand.  There are a couple in the [Python issue1580](http://bugs.python.org/issue1580) discussion, though.  In [msg58966](http://bugs.python.org/issue1580#msg58966), there's an example where MSVC gives the wrong value for something that should round to `1024.0`.  In [msg69552](http://bugs.python.org/issue1580#msg69552), there are some values near the minimum subnormal that rounded incorrectly on OS X.  There have been various bugs discovered in the underlying `dtoa.c` code from David Gay that many platforms use; mostly since fixed, but not all platforms have updated.  See http://bugs.python.org/issue7632 for some of these that were found while incorporating `dtoa.c` into Python.

Most of these really *are* corner-case bugs that are unlikely to be encountered in practice; it would be perfectly defensible to decide to carry on using `strtod` for reasons of convenience and speed.