Chop statement for floats

[cbmarini]

This PR introduces a new Chop statement that removes floating point numbers below a specified threshold.

• Its behaviour is similar to Mathematica's chop statement.
• The threshold can either be specified as a float or rational.
• It only chops floats, so rationals are untouched.

Example usage:

#StartFloat 15d
Symbol a,b,c;
CFunction f,g;
Local F = 1.0e-10*a+4.7*b+.2e-10*c+f(2,0.000001)+g(2.,1);
Chop 1.0e-10;
Argument f;
	Chop 1/10000;
EndArgument;
Argument g;
	Chop 7;
EndArgument;
Print;
.end

results in

F =
      4.7e+00*b + 1.0e-10*a + f(2,0) + g(0,1);

[coveralls]

coverage: 57.069% (+0.05%) from 57.024%
when pulling fe4a9cc on cbmarini:chop
into 01a66ec on form-dev:master.