Fix Input::Record::is_empty()

- fix + appropriate unit test
Fix LinSys::init_from_input()

- do not read from input if given Input::Record is empty
- temporary fix for new[] nothrow operator
Fix unit test for SchurComplement.

- fix the unit test
- remove last parameter of SchurComplement constructor
  (max size of detected submatrix)
... now we allow any size of diagonal blocks however it could be very slow
    to compute big inversions