Failing to compute Sv for EK80 dataset with empty filter_time

[jarlela]

ES80/EK80 raw files may not contain any Filter data when the echo sounder is operating in CW, Power/angle mode.

Example:

import echopype as ep
ed = ep.open_raw("filename.raw", sonar_model="EK80")
print(ed["Vendor_specific"]
<xarray.Dataset> Size: 27kB
Dimensions:                      (channel: 2, pulse_length_bin: 5,
                                  filter_time: 0, WBT_filter_n: 0,
                                  PC_filter_n: 0)
Coordinates:
  * channel                      (channel) <U30 240B 'GPT 00907208a0c9-1 ES38...
  * pulse_length_bin             (pulse_length_bin) int64 40B 0 1 2 3 4
  * filter_time                  (filter_time) float64 0B
Dimensions without coordinates: WBT_filter_n, PC_filter_n
Data variables: (12/14)
    frequency_nominal            (channel) float64 16B 3.8e+04 2e+05
    sa_correction                (channel, pulse_length_bin) float64 80B 0.0 ...
    gain_correction              (channel, pulse_length_bin) float64 80B 23.0...
    pulse_length                 (channel, pulse_length_bin) float64 80B 0.00...
    impedance_transceiver        (channel) int64 16B 1000 1000
    receiver_sampling_frequency  (channel) float64 16B 5e+05 5e+05
    ...                           ...
    WBT_coeffs_imag              (channel, filter_time, WBT_filter_n) float64 0B ...
    PC_coeffs_real               (channel, filter_time, PC_filter_n) float64 0B
    PC_coeffs_imag               (channel, filter_time, PC_filter_n) float64 0B
    WBT_deci_fac                 (channel, filter_time) float64 0B
    PC_deci_fac                  (channel, filter_time) float64 0B
    config_xml                   <U6607 26kB '<?xml version="1.0" encoding="u...

With this dataset, computation of Sv will fail as the algoritm in the _compute_cal function in echoype/calibrate/api.py will attempt to intersect on an empty set (around line 148).

[jarlela]

Although #1778 do fix my current problem, I'm not sure if this is the ideal solution. Is it reasonable that the algorithm uses the filter settings for computations using power/angle data at all?