Combine multiple records into a meta-sample object

[SimonGoring]

Some synthesis work uses multiple datasets and while the current objects are great it would be useful to make a meta-sampe object that would contain multiple count data for multiple sites. Basically a list that mirrors the object returned by get_download except:

• metadata: instead of metadata for a single record there would be multiple rows
• sample.meta: this is generally age data for a single site, maybe return this as a list, or return with columns site, age, uncertainty. Not sure yet.
• taxon.list: the full set of taxa in the record.
• counts: The full set of count data, except with row names identifying the core & core depth, to be associated with information in metadata.
• lab.data: same as above, except that instead of assemblage data it would be the associated lab data.

[SimonGoring]

Right now the implementation in get_download is to just make a list of sites, but I've been working on code that at least binds sites (using merge) and just appends age, site name, latitude and longitude. I think this the best option for now.

[SimonGoring]

I've been thinking about it more. I think this ought to be a user level choice for now. There is no real benefit in combining things in a pre-defined way within the package, users will value some particular data over others, so providing a 'one size fits all' method of doing this doesn't make sense without broader input.