Support setting column types by guessing types on way in

[rufuspollock]

At the moment all column are strings. Want to support numbers etc. This would also address #25.

Two possible approaches (complementary) to setting types:

1. Set type on first creation of a column by guessing from the value given (TODO: more detail on how this would work)
2. Allow setting of column type (requires support for column metadata ...) explicitly

Item (2) is now #43. This issue is about option (1).

On way in all values are cast to column type and raise error if cannot cast correctly.

Question:

• Do we support types other than sql types (e.g. json "type"?)

Why?

• Without int / float types cannot do sum or anything else
• Cannot support json data properly (see Allow nested 'objects' (and lists) in webstore columns via jsonification #25)

[pudo]

As of 48f30a2 this is now handled for JSON input. Adding support for CSV type guessing could also be done on a lower level, e.g. in the CSV file type importer (to keep it out of the database code). We may also want to make this conditional, e.g. with _type_guess=true and _types=int,int,float,varchar query parameters.