csv_import: Thousands separator works in floating point numbers

Closes issue #4322

Adds support for the thousands character in csv parser for floats.

Previously, the thousands separator character did not pass into the core floating point number parsing algorithm:
https://github.com/pydata/pandas/blob/master/pandas/src/parser/tokenizer.c#L1861

I added an argument to this function and provided a test. Now, in a file like this:
    
    A|B|C
    1|2,334.01|5
    10|13|10

Column B would import as float type.

Also related to issue #2594