change inputs to multitaper_lfp_bandpower()

i think it would be more useful to have this function take timeseries data as input and directly compute the bandpower estimates rather than needing two computations. i don't see any use case for the current function on its own. i have been using it like this:

```
import nitime.algorithms as tsa
def multitaper_lfp_bandpower(data, fs, bands, NW=3, no_log=True, nfft=None):
f, psd_mt, nu = tsa.multi_taper_psd(data.T, fs, NW, jackknife=False, low_bias=True, NFFT=nfft)
return aopy.precondition.multitaper_lfp_bandpower(f.T, psd_mt.T, bands, no_log)
```