Calculate test coverage for the package

Maybe it would be good to add a package coverage test to get an idea of missing or inchoate tests (even if it doesn't make sense to reach 100 %).

I did a test to get an idea of the results (not everything seems right: e.g. 'frun_PE.f90')

> covr::package_coverage(type = "all")
airGR Coverage: 68.78%
R/PEdaily_Oudin.R: 0.00%
R/RunModel_CemaNeigeGR4H.R: 0.00%
R/RunModel_CemaNeigeGR5H.R: 0.00%
R/TransfoParam_CemaNeige.R: 0.00%
R/TransfoParam_CemaNeigeHyst.R: 0.00%
R/TransfoParam_GR1A.R: 0.00%
R/TransfoParam_GR2M.R: 0.00%
R/TransfoParam_GR4H.R: 0.00%
R/TransfoParam_GR5H.R: 0.00%
R/TransfoParam_GR5J.R: 0.00%
R/TransfoParam_GR6J.R: 0.00%
src/frun_PE.f90: 0.00%
R/CreateCalibOptions.R: 41.04%
R/CreateInputsModel.R: 53.55%
R/PE_Oudin.R: 54.55%
R/CreateRunOptions.R: 65.72%
R/CreateIniStates.R: 68.16%
R/UtilsErrorCrit.R: 69.41%
R/TransfoParam_Lag.R: 70.59%
R/RunModel_GR6J.R: 71.26%
R/RunModel_GR1A.R: 71.67%
R/RunModel_GR4H.R: 72.29%
R/RunModel_GR5J.R: 72.29%
R/CreateInputsCrit.R: 72.43%
R/RunModel_GR2M.R: 73.08%
R/Imax.R: 74.07%
R/RunModel_GR5H.R: 74.19%
R/DataAltiExtrapolation_Valery.R: 76.09%
R/Utils.R: 77.14%
R/RunModel_CemaNeigeGR6J.R: 77.56%
R/RunModel_CemaNeigeGR4J.R: 78.29%
R/RunModel_CemaNeigeGR5J.R: 78.29%
R/RunModel_GR4J.R: 78.31%
R/ErrorCrit_KGE2.R: 79.31%
src/frun_GR5H.f90: 79.41%
R/RunModel_CemaNeige.R: 79.59%
R/plot.OutputsModel.R: 80.37%
R/SeriesAggreg.list.R: 83.19%
R/RunModel_Lag.R: 83.33%
R/SeriesAggreg.data.frame.R: 84.56%
R/UtilsSeriesAggreg.R: 87.50%
R/ErrorCrit_KGE.R: 87.76%
R/TransfoParam_GR4J.R: 88.00%
src/utils_D.f90: 88.57%
R/Calibration_Michel.R: 89.43%
R/Calibration.R: 90.00%
R/ErrorCrit_RMSE.R: 90.48%
R/ErrorCrit_NSE.R: 91.30%
R/ErrorCrit.R: 93.88%
src/frun_GR6J.f90: 98.35%
R/RunModel.R: 100.00%
R/SeriesAggreg.InputsModel.R: 100.00%
R/SeriesAggreg.OutputsModel.R: 100.00%
R/SeriesAggreg.R: 100.00%
R/TransfoParam.R: 100.00%
src/airGR.c: 100.00%
src/frun_CEMANEIGE.f90: 100.00%
src/frun_GR1A.f90: 100.00%
src/frun_GR2M.f90: 100.00%
src/frun_GR4H.f90: 100.00%
src/frun_GR4J.f90: 100.00%
src/frun_GR5J.f90: 100.00%
src/utils_H.f90: 100.00%