Added parameter float_precision to CSV parser

closes #8002.
@mdickinson I think would be ideal if we could do exactly what Python does here (only in 'round_trip' mode though), assuming is speed-wise equivalent. I think originally @wesm put in the xstrtod code to alleviate the speed issue (at the cost of some marginal accuracy). This PR (in master now), but @amras1 add these different modes, so I think ok to have use the Python method (that returns a c-double though). @amras1 pls check it out and see if this works. @mdickinson can you point to some examples that this would work differntly? (e.g. or is it 32/64/os that might be implemented diffently?)