Fix FilterAlgorithm.align_trace() method and unit tests

The method FilterAlgorithm.align_trace() pre-processes the trace.data input array for subsequent processing by FilterAlgorithm.firfilter() such that the first input array element corresponds to a time step on which output samples must fall (as defined in the dictionary step) minus half the fir window width. In short, it ensures that output samples fall on desired time steps.

Prior to this fix, it only worked as intended when input trace's starttime fell on an even time step. A bug became obvious when attempting to filter data from non-Geomag stations that did not have nice time stamps. This fix addresses that issue, and also ensures that align_trace() does what was claimed in its own original docstrings, which is to handle trailing misalignments as well.

Note: one thing align_trace() does NOT do is ensure that all needed input data are available to generate desired outputs. The user is responsible for providing this, but can use the FilterAlgorithm.get_input_interal() method to calculate the actual required input starttime and endtime. The method align_trace() will trim or pad with NaNs only enough to align time stamps, and may actually result in firfilter() output that are NaNs if the input trace was not adequate.