extract_masses: Extract known masses from a peak-list.
In Armadilloa16/dipps: Tools for Bruker Peaklist Format MSI Data

Typically used on a peak-list as produced by combine_peaklists to extract peaks within a given tolerance of a set of known masses.

1 2	extract_masses(df.peak, masses, margin = 0.3, use_ppm = FALSE, var = "m.z")

`df.peak`	A data.frame with column named `var`.
`masses`	A numeric vector of masses to extract.
`margin`	A positive number: the tolerance around `masses` to extract.
`use_ppm`	A logical value indicating if `margin` is given in ppm, if not `margin` is assumed to be in the same units as `var`.
`var`	A character string representing the variable/ column in `df.peak` that contain the values to be subsetted.