add flag for specifying the (temporal) direction of correlations

[jeremymanning]

• should be passed to timecorr and wcorr
• options: right, left, or both (default)
• behavior:
  • if both, leave weights unchanged
  • if right, set all weights for past timepoints (i.e. times < t) to 0
  • if left, set all weights for future timepoints (i.e. times > t) to 0

this will allow us to examine directional correlations