Feature: support more specific datatypes

[jflairie]

Is your feature request related to a problem? Please describe.
As of now numerical columns are imported with a lot of decimals, even when not needed. (ex: 0.8 will be imported as 0.800000000000000000). It decreases the readability and consumes unnecessary storage space on the database. Similarly, we can do the same with other data types like varchar.

Describe the solution you'd like
Add the ability to specify more specific datatypes in the sheets.yml configuration.

[bastienboutonnet]

This should be very doable! As of version >=1 we are using SQL Alchemy data types which should make imposing this pretty easy. I would first tackle the numerical aspect and then the varchar.

I think it might also be good to implement support for INTs as we currently don't have a type for this.

Most of the magic is likely to happen in: https://github.com/bastienboutonnet/sheetwork/blob/dev/objekt/core/adapters/impl.py