allow for read_airports( type = 'all')

[rafapereirabr]

No description provided.

[rafapereirabr]

The data sets of private and public airports have fairly different structures with different numbers of coulmns, columns with different names, and in a few cases the same information reported in different ways. Allowing the function to return all airports would ideally require to hamornize all columns in the two data sets in order to pile them up.

for a short term solution, the package will only return the columns found in common on both data sets.