modlib: HCV viral dynamics models

## Models

`viral1`

: viral dynamics model with single HCV species`viral2`

: viral dynamics model with wild-type and mutant HCV species

## Parameters

`s`

: new hepatocyte synthesis rate (cells/ml/day)`d`

: hepatocyte death rate constant (1/day)`p`

: viral production rate constant (copies/cell/day)`beta`

: new infection rate constant (ml/copy/day)`delta`

: infected cell death rate constant (1/day)`c`

: viral clearance rate constant (1/day)`fit`

: mutant virus fitness`N`

: non-target hepatocytes`mu`

: forward mutation rate`Tmax`

: maximum number of target hepatocytes (cells/ml)`rho`

: maximum hepatocyte regeneration rate (1/day)