Speedup PartialPivLU for small matrices by passing compile-time sizes when available.

This change set also makes a better use of Map<>+OuterStride and Ref<> yielding surprising speed up for small dynamic sizes as well.
The table below reports times in micro seconds for 10 random matrices:
           | ------ float --------- | ------- double ------- |
     size  | before   after  ratio  |  before   after  ratio |
fixed	  1	 | 0.34     0.11   2.93   |  0.35     0.11   3.06  |
fixed	  2	 | 0.81     0.24   3.38   |  0.91     0.25   3.60  |
fixed	  3	 | 1.49     0.49   3.04   |  1.68     0.55   3.01  |
fixed	  4	 | 2.31     0.70   3.28   |  2.45     1.08   2.27  |
fixed	  5	 | 3.49     1.11   3.13   |  3.84     2.24   1.71  |
fixed	  6	 | 4.76     1.64   2.88   |  4.87     2.84   1.71  |
dyn     1	 | 0.50     0.40   1.23   |  0.51     0.40   1.26  |
dyn     2	 | 1.08     0.85   1.27   |  1.04     0.69   1.49  |
dyn     3	 | 1.76     1.26   1.40   |  1.84     1.14   1.60  |
dyn     4	 | 2.57     1.75   1.46   |  2.67     1.66   1.60  |
dyn     5	 | 3.80     2.64   1.43   |  4.00     2.48   1.61  |
dyn     6	 | 5.06     3.43   1.47   |  5.15     3.21   1.60  |

0 files changed, 0 insertions, 0 deletions