feat(loading): robust error reporting and mutation

cfg now reports which fields are missing when loading. Further, the mutate method has been added for easier mutation of the underlying configuration struct

tests/config/example_config_files/example.good.missing.field.toml

@@ -0,0 +1,16 @@
+[main]
+description = "This is an example configuration file."
+author = "Example Author"
+
+[main.physics]
+diffusion = true
+convection = false
+radiation = true
+flags = [1, 0, 1]
+
+[main.simulation]
+output_frequency = 10
+
+[main.output]
+format = "csv"
+directory = "results/"

tests/config/example_config_files/example.missing.nondefault.field.toml

@@ -0,0 +1,15 @@
+[main]
+description = "This is an example configuration file."
+author = "Example Author"
+
+[main.physics]
+flags = [1, 0, 1]
+
+[main.simulation]
+time_step = 0.01
+total_time = 100.0
+output_frequency = 10
+
+[main.output]
+format = "csv"
+directory = "results/"