The FLOAT gate in testFloat allowed only a minus sign on the mantissa, while
the exponent right after it already allowed either sign. dynamicTyping
therefore left +42 and +1.5E-03 as strings and converted -42 and -1.5E-03,
silently producing columns that mix numbers and strings depending on how each
row wrote its sign.
parseFloat already accepts a leading plus and returns the right value, so the
regex was stricter than the conversion it guards. Allow either sign on the
mantissa, matching the exponent.
The 2^53 precision guard is unchanged, and a plus that is not a sign is still
rejected.
Fixes#1070
