Equivalent width measurements

[jnburchett]

trafficstars

In the spectral windows, we want to enable a measurement mode where the user can quickly measure the equivalent width of a line by clicking (1) the left-hand starting pixel, (2) the right-hand ending pixel, then (3) the approximate centroid of the line to get a reference redshift. The following should then get stored for each measurement: EW, EW error, ACD, ACD error, starting wavelength, ending wavelength, reference redshift, rest wavelength (1215.67 for HI, 1548.195 for CIV), skewer ID.

Note that that EW and ACD and their respective errors are output from the measurement routines provided by JNB.