sero_det: Create a sero detection probability density function
In mrc-ide/india-ascertainment: Analysis of COVID-19 mortality in India

Create a sero detection probability density function

sero_det(
  ret = "igg",
  igg_conv = 13.3,
  igm_conv = 12.3,
  igg_sens = 0.914,
  igm_sens = 0.894,
  igg_scale = 139,
  igm_scale = 50,
  pdfl = 600
)

`ret`	What antibody are we detecting. Must be one of "igg", "igm" or "iggm". Default = "igg
`igg_conv`	Mean days for IgG seroconversion. Default = 13.3
`igm_conv`	Mean days for IgM seroconversion. Default = 12.3
`igg_sens`	Max sensitivity for IgG. Default = 0.914
`igm_sens`	Max sensitivity for IgM. Default = 0.894
`igg_scale`	Weibull seroreversion decay scale for IgG. Default = 139
`igm_scale`	Weibull seroreversion decay scale for IgM. Default = 50
`pdfl`	Length of probability density function. Default = 600 days