Refactoring / defenitions of gaps

[vergauwenthomas]

The gap and missing obstype modules are becoming too complex. It is time to refactor them. The idea is to merge the gaps and missing obstype together into the new gap$ class.

Gap attributes:

• [ ] startdt, enddt (in original import freq)
• [ ] obstype
• [ ] station
• [ ] gapdf (only the gap records!)
• [ ] anchordf (only the observation records used, trailing/leading)

gapdf (and anchordf) is a dataframe with datetime-name index (easier for merging), and the following columns:

• "obstype.name" column is always filled with Nan's
• "fill": the filled values
• "fill method" : method for filling
• "msg": error/info messages per record

The following issues must be resolved here

• 431

• 417

• 179

• coming requests/remarks by @amberJ99

[vergauwenthomas]

obsolete (duplicate of #467)