Enhance search_eurostat(), TOC handling, documentation

[pitkant]

• Use inheriting in documentation where possible to avoid things being outdated in some parts of the package
• Include NEWS.md in the R package build

[pitkant]

• Allow search_eurostat() to search from TOC codes in addition to titles by adding new column argument (as described in issue add code search to search_eurostat  #86 )
• Replace empty spaces in eurostat TOC object names (column names) with ., making subsetting TOC easier

[pitkant]

• Add hierarchy column to TOC, possibly making it possible to download whole folders with a single command in the future (as described in issue get_eurostat folder handling #226)

[pitkant]

• New function get_eurostat_folder() makes it possible to download several datasets with one function. Solves issue get_eurostat folder handling #226

[pitkant]

• Update documentation on dataset filtering and start using new URL for Eurostat dictionaries

[pitkant]

• Previous push broke some functions that needed to be either rewritten or given a temporary fix.
• Rewrite label_eurostat_tables()
• Deprecate label_eurostat_vars() (meaning it still works)
• Create a temporary function label_eurostat_vars2() that will replace label_eurostat_vars() when it stops working. Do not use label_eurostat_vars2() in any scripts, it's mainly for testing purposes
• Testing fetching dataset metadata from SDMX API SDMX-ML files, might be worthwhile to use this approach more widely as replacements to label_eurostat and get_eurostat_dic functionalities