Add function to discard kspace locations out-of-bounds, and related tests

[Daval-G]

As opposed to previous PR #128, kspace samples located outside the [-0.5; 0.5[ limits should simply be discarded. Since this manipulation is not expected from normalize_kspace_locations and would represent an unwanted and redundant intermediate step inside the Fourier operators, it was preferred to simply add an utilitary function for the users.

Additional tests were added in mri/tests/test_fourier_adjoint.py for 2D and 3D by mimicking the normalize_frequency_locations tests. However, the latters had issues in their assertions and all four tests were properly updated to match the intended checks.

[chaithyagr]

I just submitted my change @Daval-G , LGTM, merge when green