Implement an optional check for positive definiteness of shape field of ellipsoid class

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Many methods of ellipsoid class use ell.shape= notation to assign values for 
shape property even when ell object is not the one for which the method is 
called. Thus the ellipsoid class constructor is not the only entry point for 
shape property value. As a result, an incorrectly implemented method may 
construct an ellipsoid with incorrect value of shape property (not-symmetrical, 
not positive definite etc). 

We need to incorporate an optional check for correctness of shape property into 
set.shape property set method (see 
http://www.mathworks.com/help/matlab/matlab_oop/property-access-methods.html#brg
sek9-3)

In this methods we need to use elltool.conf.Properties.getIsDebug value to 
determine whether to perform the check. A proper checking would require use of 
gras.la.ismatposdef function which accepts absTol. 
Thus we should make sure that absTol property is already set for ellipsoid 
class prior to assigning value to shape property.

isDebug should be added to xml configurations and default value should be 
fauls. However, elltool.test.run_tests should set this value to true.

Original issue reported on code.google.com by heartofm...@gmail.com on 20 Apr 2013 at 8:54