Add list of "IssueEntry" to StudyEntry

[j-coll]

There are some events in a Variants Dataset that can be considered issues, like duplications, discrepancies, mendelian errors... Adding a list of issues at StudyEntry level could help to report them to end-users.

Proposed data model

class SampleEntry {
    union { null, string } sampleId;   // optional
    Integer                fileIndex;
    array<string>          data;  (see #179) 
}

enum IssueType {
    DUPLICATION,
    DISCREPANCY,
    MENDELIAN_ERROR,
    DE_NOVO
}

class IssueEntry {
    IssueType              type;
    SampleEntry            sample;
}

class StudyEntry {
    ...
    array<IssueEntry>      issues;
    ....
}