fixes.

which did not prevent the commit (svn r722564) to 'svn copy' a directory
called 'Core/' on top of an existing file 'Core'

see http://websvn.kde.org/?view=rev&revision=722564

* only try...catch if exceptions are enabled

the name 'PartialLU' is not meant to be definitive!
make inverse() and determinant() use it, so it's *almost* considered
well tested.

introduce ei_is_diagonal to check for it
DiagonalCoeffs ---> Diagonal and allow Index to by Dynamic
-> add MatrixBase::diagonal(int) with unittest and doc

*fix a unused variable warning

turns out to be similar to Higham's formula already in use in LDLt

 --->they go into Matrix as they resize.
*add isConstant() alias to isApproxToConstant()
*extend unit-test
*change an assert into a static assert

* test Part::swap

making fitHyperplane no longer use it)

deprecated). Basically there are now only 2 functions to set a
coefficient:
1) mat.coeffRef(row,col) = value;
2) mat.insert(row,col) = value;
coeffRef has no limitation, insert assumes the coeff has not already
been set, and raises an assert otherwise.
In addition I added a much lower level, but more efficient filling
mechanism for
internal use only.