Further simplify delphin.tsdb module

[goodmami]

For purposes of type-checking (which help to reveal actual bugs), the delphin.tsdb module could be made simpler. Mainly this has to do with function parameters that change the structure of the thing that is returned. For instance:

• delphin.tsdb.split() should always return a tuple of strings, some separate function should do the casting
• delphin.tsdb.Database should remove the autocast parameter/attribute
• delphin.tsdb.Database.select_from() should remove the cast parameter
• delphin.tsdb.join() should remove the fields parameter (maybe)

Then there would be separate functions for casting or formatting all columns in a record.

These are backwards-incompatible changes, so either they need to be done with different names or wait until v2.0.