Coerces a dataset into a echosounder dataset.
as.echosounder(
  time,
  depth,
  a,
  src = "",
  sourceLevel = 220,
  receiverSensitivity = -55.4,
  transmitPower = 0,
  pulseDuration = 400,
  beamwidthX = 6.5,
  beamwidthY = 6.5,
  frequency = 41800,
  correction = 0
)times of pings
depths of samples within pings
matrix of amplitudes
optional string indicating data source
source level, in dB (uPa at 1m), denoted sl in
reference 1 p15, where it is in units 0.1dB (uPa at 1m)
receiver sensitivity of the main element, in
dB(counts/uPa), denoted rs in reference 1 p15, where it is in units of
0.1dB(counts/uPa)
transmit power reduction factor, in dB, denoted
tpow in reference 1 p10, where it is in units 0.1 dB.
duration of transmitted pulse in us
x-axis -3dB one-way beamwidth in deg, denoted bwx
in reference 1 p16, where the unit is 0.2 deg
y-axis -3dB one-way beamwidth in deg, denoted bwx
in reference 1 p16, where the unit is 0.2 deg
transducer frequency in Hz, denoted fq in reference 1 p16
user-defined calibration correction in dB, denoted
corr in reference 1 p14, where the unit is 0.01dB.
Creates an echosounder file.  The defaults for e.g.  transmitPower
are taken from the echosounder dataset, and they are unlikely to make
sense generally.
Other things related to echosounder data: 
[[,echosounder-method,
[[<-,echosounder-method,
echosounder-class,
echosounder,
findBottom(),
plot,echosounder-method,
read.echosounder(),
subset,echosounder-method,
summary,echosounder-method