Add taxonomy() accessory function

[damianooldoni]

This function returns the taxonomic information from observations(x).

[peterdesmet]

Two questions:

1. Should we name this taxa() instead?
2. Should we drop the taxon. prefix for the columns returned by this function?

I'm tempted to say yes to both.

[damianooldoni]

1. About function name: I understand why you prefer taxa(): it's a plural name as locations(), observations()etc. So, I agree 👍
2. In camtraptor's read_camtrap_dp() no prefixes were used, so removing the prefix taxon. will make our life while refactoring camtraptor way easier 😄 The point: do we easily can identify these columns and how likely there will be conflicts among taxonomic related columns and observations original columns in the future? I think it will be very unlikely. So, 👍 to remove the prefix.