inverse operation to def __str__()

I find it a common requirement to parse strings previously obtained via the default string representation. The parsing and formatting routines should be semantically inverse.
IMO, the library should contain therefore a possibility to parse values without adding a default uncertainty of +/-1 in the last digit.
This could be added as a separate parameter to ufloat_fromstr in order to not break the API. I found it simpler to add an extra routine ufloat_parse.
Let me know, what you think of this.