[essnmx] Wavelength range study

We thought NMX would operate within a single wavelength range that fits within 1 pulse window,
$1.8 å , 3.55 å$
but instrument scientist wanted to add at least one more wavelength range,
$2.2 å , 3.95 å$

We will first have to

• Study the chopper settings for each mode

and then we have to decide ...

• If we want to use pre-computed look up table or use tof to compute it on the fly
• How do we allow users to select wavelength range
  Right now, you can give essnmx-reduce arbitrary wavelength range, but it'll be nice to have a way to select the range automatically from file or by selecting one mode from multiple choices.