Improve datafacer validation

[padeg]

Some imperfect datafacer declarations rise compilation errors while that could be detected at validation time and just notified to the user.

Two examples :

• writing data CsvFile(...) instead of data Csvfile(...)
• Writing data Shapefile("file.shp",2975) instead of data Shapefile("file.shp","EPSG:2975")

[padeg]

Fixed the first case : we check if the type of the datafacer does exist.
Validation of the function signature should be added later.