Fix warning about double/float conversion in kReciprocal() kernel

When compiling cudamat, I always get a warning:
```
ptxas info    : 0 bytes gmem
ptxas info    : Compiling entry function '__cuda_dummy_entry__' for 'sm_10'
ptxas info    : Used 0 registers
ptxas /tmp/tmpxft_00003909_00000000-10_cudamat_kernels.ptx, line 5212; warning : Double is not supported. Demoting to float
```
By pressing CTRL+Z during make and opening the mentioned temporary file, I found that this is because of the double precision constant in `kReciprocal()`. Replacing it with a float solves the issue.

For GPUs with higher compute capability, we could also use a double there, for increased precision... this would need some changes to the Makefile, though.