Fixed Significant digits with both exponent and scientific calculators. ...

[vikramahuja1001]

...Fixes #4814.

The concept of Significant digits was broken in calculate. There were also trailing zeros in the calculate result. Eg: ''' 11X100 ''' used to return '''1100.00''', '''1+1''' -> ''' 2.00000''', '''sin(180)''' -> ''' 10^-16''' .
Exponent Calculator returns the result in form of aX10**b and scientific calculator returns normal result. Changed the number of significant digits from 6/9/12/15 to 3/6/9. Fixed the errors by changing format_decimal function in mathlib.py. Both Exponent and scientific calculators now respect the number selected in the number of significant digits in the toolbar.

[godiard]

@vikramahuja1001, this is a different version of the patch already merged? Can we close?

[vikramahuja1001]

@godiard The patched merged initially was not complete. This is the latest one. I think it should be tested for all the cases provided in the discription.

[quozl]

Please rebase and resubmit, thanks.