TextLoader should have only one constructor, that doesn't take Arguments as a parameter

[markusweimer]

TextLoader needs to hide the constructor that has Arguments, and also expose the HasHeader and SeparatorChars to be non-advanced parameters.

-- old description --
TextLoader presently accepts a parameter called column for the array of columns to read. That seems odd, given that almost always, multiple columns are passed. Hence, it would be more natural to call that parameter columns.

[markusweimer]

/cc @Zruty0

[Zruty0]

@markusweimer that's not exactly correct. TextLoader takes Arguments as a parameter, and the arguments class has a field called Column. This is actually a common pattern in our codebase to use singular nouns for things that are internally arrays: it's better in command line to have
loader=Text{column=Foo:TX:0 column=Bar:R4:1}
than it is to have
loader=Text{columns=Foo:TX:0 columns=Bar:R4:1}.

However, I don't think the constructor that takes TextLoader.Arguments needs to be public altogether. We should improve the other constructor to have non-advanced parameters, and also do #1515 .

[Zruty0]

I took a liberty to update the issue name and description. I kept the old one for history

[eerhardt]

See dotnet/apireviews#81 for some notes from the API design discussion on this API. In the end, we decided to have 2 constructor/factories:

1. With the simple/common options as direct parameters.
2. If a component needed to have public "advanced" options, have another constructor/factory that took only the Options instance.