getVarCov() fails with what I think is due to fixed parameters and parameter near boundary error

[billdenney]

Related to #548 in that I'm stopping trying to make the simulation work for now, and I'm going to describe the issue that I was trying to get to without a direct reprex.

While estimating a model, I got the warning at the end:

6: In (function (uif, data, est = NULL, control = list(), ..., sum.prod = FALSE,  :
  parameter estimate near boundary; covariance not calculated
 use 'getVarCov' to calculate anyway

I didn't see which parameter appeared to be near its boundary, so I assume that it is one of the fixed parameters (see the model in #548 and the request in #544).

So, I called getVarCov(model_linear_iiv), and that resulted in the following error:

> getVarCov(model_linear_iiv)
> creating full model...
> pruning branches (`if`/`else`)...
√ done
> loading into symengine environment...
√ done
> creating full model...
> pruning branches (`if`/`else`)...
√ done
> loading into symengine environment...
√ done
> calculate jacobian
[====|====|====|====|====|====|====|====|====|====] 0:00:00 

> calculate sensitivities
[====|====|====|====|====|====|====|====|====|====] 0:00:00 

> calculate d(f)/d(eta)
[====|====|====|====|====|====|====|====|====|====] 0:00:00 

> calculate d(R^2)/d(eta)
[====|====|====|====|====|====|====|====|====|====] 0:00:00 

> finding duplicate expressions in inner model...
[====|====|====|====|====|====|====|====|====|====] 0:00:00 

> optimizing duplicate expressions in inner model...
[====|====|====|====|====|====|====|====|====|====] 0:00:00 

> finding duplicate expressions in EBE model...
[====|====|====|====|====|====|====|====|====|====] 0:00:00 

> optimizing duplicate expressions in EBE model...
[====|====|====|====|====|====|====|====|====|====] 0:00:00 

> compiling inner model...
√ done
> compiling EBE model...
√ done
Needed Covariates:
[1] "TUMORBL"
Error in .fitFun(.ret) : theta mismatch op_focei.ntheta 9, ntheta: 6

Error in foceiFit.data.frame0(data = .dat, inits = .inits, PKpars = .uif$theta.pars,  : 
  Could not fit data.

... and I'm done adding new issues for now. :)

[mattfidler]

Thanks @billdenney