Invalid floating point numbers cause InvalidOperationException

[FabianNitsche]

First of all: Thanks for the very good and performant mathematical formula parser.

When only entering "." as a formula or any other invalid floating point number like "..", "..1", "0..1" the TokenReader will skip this, causing the AstBuilder to throw an InvalidOperationException because it expects the token list not to be empty. The token reader should throw an ParseException in this case.