Add HWPF .NET Core support to read Word / Excel 2003 ( .doc / .xls) forma

[RockNHawk]

Add HWPF .NET Core support to read Word / Excel 2003 ( .doc / .xls) format

Fix HWPF multi-thread bug : System.InvalidOperationException: Collection was modified; enumeration operation may not execute.

[yang-xiaodong]

Hello , thanks your PR~

Are you missing some unit test about HWPF support, and all passed ?

In addition, we need to both support the .net standard 2.0 and .net framework 4.6.1, because someone are stills using .net framework now , can you do it ?

[RockNHawk]

Hello , thanks your PR~

Are you missing some unit test about HWPF support, and all passed ?

In addition, we need to both support the .net standard 2.0 and .net framework 4.6.1, because someone are stills using .net framework now , can you do it ?

I'm sorry about I forget the .NET Framework version... recently I have no such time to do this work, I will add the .NET Framework and unit test in the feature.

I have tested HWPF in my application it's work well (except .ppt), so I just pull it if someone needs use NPOI in .NET Core to read Office 2003 can use it or find it and compile it by self.