Add receiver- and source-side corrections

[anowacki]

We should add the option to do both of the following:

1. Pre-correct the traces with a 'receiver-side' correction—that is a splitting operator which should be removed from the traces before measuring the splitting, recovering what should be the splitting before this correction. This is straightforward and can already be done manually.
2. Post-correct the traces with a 'source-side' correction—that is, a splitting operator should be removed from the traces after the trial splitting operators at each grid point of the search, determining the eigenvalues of the particle motion that would result from only the splitting after the correction. This is harder and has to be done within the grid search so cannot be done by the user manually.