rasqal_format_double use fixes

(rasqal_xsd_format_integer): Use rasqal_format_double with 7
significant digits for float.

(rasqal_xsd_format_double):  Define number of siginficant digits
constant.  Use final len from rasqal_format_double()
Alternate rasqal_format_double implementation
rasqal_double_getdigit
code tidy
(rasqal_literal_as_node): Normalize xsd:double and xsd:float strings
Add a configure test for __func__ since it's not always a macro