Sometimes datasets are provided in matrix form, with first
index corresponding to longitudes ranging from 0 to 360.
`matrixShiftLongitude`

cuts such matrices at
longitude=180, and swaps the pieces so that the dateline
is at the left of the matrix, not in the middle.

`matrixShiftLongitude(m, longitude)`

## Arguments

- m
The matrix to be modified.

- longitude
A vector containing the longitude in the 0-360 convention. If missing,
this is constructed to range from 0 to 360, with as many elements as the first index of `m`

.

## Value

A list containing `m`

and `longitude`

, both rearranged as appropriate.