ascertain_ped: Determine if a pedigree is ascertained
In SimRVPedigree: Simulate Pedigrees Ascertained for a Rare Disease

Intended priamrily as an internal function, ascertain_ped checks to see if a pedigree returned by sim_ped is ascertained.

1 2	ascertain_ped(ped_file, num_affected, ascertain_span, recall_probs = NULL, first_diagnosis = NULL, sub_criteria = NULL)

`ped_file`	An object of class `ped`. A pedigree generated by `sim_ped` or `sim_RVped`, or an object created by the function `new.ped`. See details.
`num_affected`	Numeric vector. The minimum number of disease-affected relatives required for ascertainment.
`ascertain_span`	Numeric vector of length 2. The year span of the ascertainment period. This period represents the range of years during which the proband developed disease and the family would have been ascertained for multiple affected relatives.
`recall_probs`	Numeric. The proband's recall probabilities for relatives, see details. If not supplied, the default value of four times kinship coefficient between the proband and the relative is used.
`first_diagnosis`	Numeric. The first year that reliable diagnoses can be obtained regarding disease-affection status. By default, `first_diagnosis` `= NULL` so that all diagnoses are considered reliable. See details.
`sub_criteria`	List. Additional subtype criteria required for ascertainment. The first item in `sub_criteria` is expected to be a character string indicating a subtype label and the second is a numeric entry indicating the minimum number of relatives affected by the identified subtype for ascertianment. By default, `sub_criteria = NULL` so that no additional criteria is applied. See details.