AccelStab 2.0.0

This is a major release adding a few new functions and fixing a bug.

New features

step1_down_rmse() added which allows the user to calculate the root mean squared error for their data and chosen parameters
step1_down_diagnostic() added which allows the user to plot residual diagnostic plots after fitting the model
step1_sample_mvt() added which allows the user to draw a chosen number of sample parameters from the multivariate t distribution for their own analyses
step1_down() now accepts an extra argument validation which sidelines some of the data allowing the user to save it for testing purposes
step1_plot_desc() now accepts an extra argument validation which sidelines some of the data allowing the user to save it for testing purposes

When selecting a temperature outside the data set for step1_plot_T() the colours of the prediction line and the ribbon are now consistent
When using the argument temp_pred_C within step1_down() no longer are predictions duplicated if the temperature is already in the data
Issue when no time = 0 rows present in the data and no parms provided to step1_down() now resolved