Add invalid or missing data type check during PSS/CSS inventory file read

[glemieux]

While troubleshooting an issue with erroneous plant density outputs while using inventory initialization mode with @rgknox and @JessicaNeedham today, we found that the model was silently aborting the CSS file read once it hit an invalid "NA" datatype that was mistakenly included in the CSS file. We should add a check during each line read to fail then run gracefully if the read can't parse the data.