add: getdecimal() for use within mappings

[GoogleCodeExporter]

getdecimal() is useful in mappings for calculations.
A python decimal is returned (http://docs.python.org/2/library/decimal.html)
When nothing is found (None): decimal 0 is  returned.
Non-numeric strings (including empty strings) give error.

Original issue reported on code.google.com by hjebb...@gmail.com on 31 Jul 2013 at 11:16

[GoogleCodeExporter]

Changed this: non-numeric strings also return decimal 0.
warning: this function might 'mask' errors, because when input is not-valid, it 
aalwyas return a decimal 0. 

Original comment by hjebb...@gmail.com on 1 Aug 2013 at 11:40

[GoogleCodeExporter]

Original comment by hjebb...@gmail.com on 6 Sep 2013 at 4:54

• Changed state: Fixed

[GoogleCodeExporter]

Original comment by hjebb...@gmail.com on 10 Sep 2013 at 12:46

• Changed state: Done