Added parameter float_precision to CSV parser

closes #8002.
`PyFloat_FromString` isn't deprecated!  It's very much current code.

The roundtripping of Python floats (i.e., the fact that `float(repr(x)) == x` for all finite floats `x`) depends on `strtod` being correctly-rounded.  Unfortunately, that's not a guarantee that most operating systems make for their `strtod` implementations, which is why we (i.e., the Python core devs) had to use our own implementation of `strtod` when introducing the short float repr in Python 2.7.  (All the gory details are in http://bugs.python.org/issue1580.)  There are a wide variety of interesting and exciting corner-case `strtod` bugs, varying by platform.

In practice, the platform `strtod` is probably "good enough" most of the time; I wouldn't expect to run into those corner cases very often.  So if the occasional (platform-dependent) failure to roundtrip in corner cases is acceptable then `strtod` should be fine.  If you want guaranteed roundtrip behaviour, it's better to use the same code that Python uses for converting strings to floats.

The decision of what's acceptable is of course your choice!