See #24

Reformatted getNonZeroRowLength to pass on the device Type in string and thus enable the lambda in SparseRow_impl.h

Fixed POSSIBLE MISTAKE in both addElement functions, where columns where being compared to this->rows, changed the formed to be compared to this->columns.

Reformatted getNonZeroRowLength to pass on the device Type in string and thus enable the lambda in SparseRow_impl.h

Fixed POSSIBLE MISTAKE in both addElement functions, where columns where being compared to this->rows, changed the formed to be compared to this->columns.

The former has been formally deprecated for a long time and in CUDA 10.0
it is finally properly marked as deprecated, so the compiler can print
warnings.

The x and b vectors might have different type.

- base class with non-virtual destructor causes undefined behaviour
- missing return statements at the end of non-void functions
- missing const before char* function argument
- reordered initialization of class members
- misleading indentation after for statement
- fixed use of uninitialized variables
- fixed unsequenced modification and access to variables (the C++
  standard does not define the evaluation order of operands, so
  expressions with side-effects such as j++ cause undefined behaviour)