allowing international letters like æøå

[samuelsen]

Ref issue: #11 (comment)
In the latest version of the library the following code has been introduced to validate input names:

Common.DMN.Engine/net.adamec.lib.common.dmn.engine/engine/definition/DmnVariableDefinition.cs

Line 136 in 524e55e

public static string NormalizeVariableName(string name)

For out part this brakes because of using Norwegian letters like æ, ø, å, within the DMN files, and also as input params to the library.

A better solution would probably be to use the .net function:

public static string NormalizeVariableName(string name)
{
    var retVal = name?.Trim().Replace(' ', '_');

    if (string.IsNullOrWhiteSpace(retVal))
        throw new ArgumentException($"Variable name is null or empty",nameof(name));

    if (retVal.Any(c => char.IsLetter(c) || char.IsDigit(c)  || c == '_')))
        throw new ArgumentException($"Variable name '{name}' contains invalid character", nameof(name));

    if (!char.IsLetter(retVal[0])
        throw new ArgumentException($"Variable name '{nameof(name)}' must start with letter", nameof(name));

    return retVal;
}

[adamecr]

Hello Simen, the idea seems to be ok, there are just some minor syntax issues in the PR (see the comments).
When I did the corrections, it compiles fine and it also pass all tests. I'll think a bit about possible impacts, but so far don't see any

[adamecr]

@samuelsen thanks for the update, it's passing the compilation and tests fine now

[adamecr]

I'm building some additional tests to ensure the proper/expected behavior when using the non-ASCII letters in input and variables before merging the PR (will report the progress at related issue #11 )

cc @samuelsen