Add zoom mode to LOW sensitivity calculator

This feature adds sensitivity calculation for zoom/spectral line observations to the LOW sensitivity calculator.

Frontend:

Create a new accordion with Zoom as its title.

With SKA LOW, zoom observational setup is determined by two parameters:

1. Zoom mode - A dropdown button that allows users to select one of the eight standard modes in which a LOW zoom window can be configured. Each mode has a fixed channel resolution and bandwidth. The channel resolution of each zoom mode is given by the relation:

   (2**(N-1))*(781250.*32/27)/4096/16

   Here N is the zoom mode and takes values from 1 to 8 (The relation comes from Grant H. via Andy B.). Each zoom window contains 1728 channels, so the bandwidth is given by 1728*channel width. 

2. The central frequency of the zoom window.
3. Channel averaging (integer less than or equal to half the number of zoom channels, which is 1728/2 = 864).

For the given set of input parameters, the frontend should display the per-channel sensitivity at the center of the zoom window (after channel averaging). 

Each accordion should contain the following acknowledgement. This was missed in SP-3171, and it would be good to add this message to the continuum accordion too. In discussions with Andy, it was agreed that this text could appear in a single place so long as this was always visible. The footer seems like a possibility.

"Note: The theoretical sensitivity is computed using direction, frequency, and LST-dependent SEFD values following the procedure described in Sokolowski et al. (2022)." Link the text "Sokolowski et al. (2022)" to https://doi.org/10.1017/pasa.2021.63
 
Backend:

The backend should compute the theoretical sensitivity for a representative channel (taken to be at the band centre). This feature should probably not require many changes to the backend. We can repurpose what has been created for the continuum sensitivity calculation.