add covariate on trait/edit

see <a href="https://ebi-forecast.igb.illinois.edu/redmine/issues/2198"> redmine #2198</a>
    When I suggested using save! instead of save, it was because it
    was the only way I could think of to get the true error message
    back from the code that triggered the error.  What I didn't like
    about your previous code is that any error whatsoever that caused
    saving of the covariate to fail would get reported as an "out of
    range" error.  Moreover, the error message that comes from the
    validation code contains more information: it tells what the
    minimum allowable value is if the supplied value is too low and
    what the maximum allowable value is if the supplied value is too
    high.

    I have since discovered there is a better way to get error
    messages back from the validator.  If you call @covariate.valid?
    or @covariate.invalid?, this will call the Rails validator and set
    the @covariate.errors property, which will then contain the
    information from the validator.

    So I now think that because the Rails validator will usually give
    us the information we want without triggering an exception, we
    should only use exception handling in the case where the error
    comes from the database itself (see the suggested code I put in
    the Redmine issue).