This function takes in the mixture data, food source means and standard
deviations, and (optionally) correction factor means and standard
deviations, and concentration proportions. It performs some (non-exhaustive)
checking of the data to make sure it will run through simmr. It outputs an
object of class `simmr_input`

.

```
simmr_load(
mixtures,
source_names,
source_means,
source_sds,
correction_means = NULL,
correction_sds = NULL,
concentration_means = NULL,
group = NULL
)
```

- mixtures
The mixture data given as a matrix where the number of rows is the number of observations and the number of columns is the number of tracers (usually isotopes)

- source_names
The names of the sources given as a character string

- source_means
The means of the source values, given as a matrix where the number of rows is the number of sources and the number of columns is the number of tracers

- source_sds
The standard deviations of the source values, given as a matrix where the number of rows is the number of sources and the number of columns is the number of tracers

- correction_means
The means of the correction values, given as a matrix where the number of rows is the number of sources and the number of columns is the number of tracers. If not provided these are set to 0.

- correction_sds
The standard deviations of the correction values, given as a matrix where the number of rows is the number of sources and the number of columns is the number of tracers. If not provided these are set to 0.

- concentration_means
The means of the concentration values, given as a matrix where the number of rows is the number of sources and the number of columns is the number of tracers. These should be between 0 and 1. If not provided these are all set to 1.

- group
A grouping variable. These can be a character or factor variable

An object of class `simmr_input`

with the following elements:

- mixtures
The mixture data

- source_neams
Source means

- sources_sds
Source standard deviations

- correction_means
Correction means

- correction_sds
Correction standard deviations

- concentration_means
Concentration dependence means

- n_obs
The number of observations

- n_tracers
The number of tracers/isotopes

- n_sources
The number of sources

- n_groups
The number of groups

For standard stable isotope mixture modelling, the mixture matrix will
contain a row for each individual and a column for each isotopic value.
`simmr`

will allow for any number of isotopes and any number of
observations, within computational limits. The source means/sds should be
provided for each food source on each isotope. The correction means (usually
trophic enrichment factors) can be set as zero if required, and should be of
the same shape as the source values. The concentration dependence means
should be estimated values of the proportion of each element in the food
source in question and should be given in proportion format between 0 and 1.
At present there is no means to include concentration standard deviations.

See `simmr_mcmc`

for complete examples.

```
# A simple example with 10 observations, 2 tracers and 4 sources
data(geese_data_day1)
simmr_1 <- with(
geese_data_day1,
simmr_load(
mixtures = mixtures,
source_names = source_names,
source_means = source_means,
source_sds = source_sds,
correction_means = correction_means,
correction_sds = correction_sds,
concentration_means = concentration_means
)
)
print(simmr_1)
#> This is a valid simmr input object with
#> 9 observations,
#> 2 tracers, and
#> 4 sources.
#> The source names are:
#> [1] "Zostera" "Grass" "U.lactuca" "Enteromorpha"
#> .
#> The tracer names are:
#> [1] "d13C_Pl" "d15N_Pl"
#>
#>
```