Correct the Documentation of Template Duration in svd_bank

We discuss in O4-code-review that the template_duration in svd_bank should be documented as the exact time of waveform, instead of an estimated value from f-low. Basically, lalsimulation slightly adjust the f-low range to match the waveform generation, which results a slightly-longer waveform in time domain. We adjust the template duration to this value.

Also, nicely, I fix the spin1 and spin2 calculation in svd_bank.py and gstlal_inspiral_bank_splitter using numpy.dot.