fix: Number Column With Fixed Size

[jduchateau]

Fix the situations were this FlatFileDefinition would throw an InvalidOperationException(if a size is given, all sizes must be given).
Solve it and add to the exception the indexes of the columns without a specified size.

FlatFileDefinition.Create(i => new {
    first = i.ToNumberColumn<double>(0, 19, "."), // Size not taken into account
    second = i.ToColumn(1, 5),
});

Targeting master, increase minor version number to 2.1.17-beta.