Feature/split gauge reuse

[aroadant]

These modifications will allow the reuse of the redistributed gauge buffers.

QudaGaugeParam.use_split_gauge_bkup is added to determine whether to delete the buffer after use or not. Its default value is 0. The internal flag update_split_gauge is used to determine whether an update of buffer is needed or not.

The initial test with staggered_invert_test.cpp gives the same results with gauge_param.use_split_gauge_bkup = 0 or 1. Also inversions can be done correctly with inv_param.split_grid[i] = 1 after split gauge inversions. Results from various default tests are good with default value use_split_gauge_bkup=0/1.

The test staggered_invert_test is updated with the flag --use_split_gauge_bkup. Results from the following input looks good

staggered_invert_test --dim 6 6 6 6 --gridsize 2 2 2 2 --grid-partition 2 2 2 2
-dslash-type staggered --compute-fat-long false --unit-gauge 0 --tadpole-coeff 1.0 --verbosity-precondition debug --recon 13
--mass 0.02 --solve-type direct-pc --solution-type mat --inv-type cg --matpc even-even
--prec double --prec-sloppy single
--nsrc 64 --nsrc-tile 16 --niter 10000 --tol 1e-100
--inv-multigrid false --mass-normalization mass --fermion-t-boundary periodic --verify 1 --use_split_gauge_bkup 1