Refactor Shift for efficiency

[dcookman]

Following in the footsteps of the efficiency gains made in PR #25 for the Scale systematic, the same approach has been applied to the Shift systematic.

In order to construct the shift systematic response matrix, we:

1. Cache the bin mapping between elements of the matrix in the full space to the shifting subspace once at the start;
2. Construct the shifting response matrix in the 1D shifting axis only;
3. Use the bin mapping to create the full response matrix.

I have also moved the contents of the calculation of the fractional bin contribution for a given element into its own private method, using the same code as in Scale - so the bug fix there is included here too.