build_metadata: Create blank metadata
In ressy/srahelper: SRA Helper

Create a new empty SRA metadata data frame with columns named and typed accordingly.

1 2	build_metadata(sample_attrs, submission = NULL, col_pairs = NULL, constants = NULL)

`sample_attrs`	data frame of existing sample metadata to draw from. Any names given in the `col_pairs` argument will be used to explicitly map column names from the existing data frame to the new data frame. Remaining columns with matching names will also be used. Other columns not in `col_pairs` or the template's fields are ignored.
`submission`	the accession assigned by the submission portal for the submission, like "SUB####". Will be attached to the output data frame as an attribute.
`col_pairs`	named vector of column names in the existing data frame with names set to column names in the new data frame. Vector names that don't match known column names signify custom columns to add.
`constants`	vector of field names to match to constant values for all samples.