Allow data.frame input to load_[mix/source/discr]_data and add option print/save controls to isospace plot_data

[hannahrempel]

I made two related sets of modifications to improve usability when running many MixSIAR models programmatically.

1. Allow data.frame input to load_mix_data(), load_source_data(), and load_discr_data()

I lightly modified the load_[mix/source/discr]_data functions to accept data.frame objects in addition to file paths to .csv files.

Reason: This allows users to format MixSIAR input data within R scripts, rather than writing multiple intermediate .csv files and reading those into the models. This is particularly helpful when running many models (e.g., multiple consumer species with species-specific informative priors–necessitating an individual model per species), where the current workflow would require generating dozens of separate .csv files (e.g., 10 species × mix/source/discrimination = 30 files). Accepting data.frame input makes large, scripted workflows more efficient and user-friendly.

2. Add optional control over saving, printing, and returning isospace plots in plot_data()

I modified plot_data() to optionally:

• save plots to file,
• print plots to the active graphics device, and/or
• return ggplot objects for modification.

Reason: This provides users with more control over plotting behavior and helps prevent graphics device overload—particularly in R Markdown or batch workflows—when running multiple models or using many tracers. For example, models with six EAA tracers produce 15 pairwise isospace plots per model; when run in loops/across multiple species, this can easily trigger “too many open devices” errors. Separating plot saving from plot display improves stability without changing default behavior.