Bump version to 2.10.0

Update test project to enable Live Unit Testing in VS 2017

The Live Unit Testing (LUT) feature was introduced in Visual Studio
2017 Enterprise Edition version 15.3.

NuGet packages added to the test project:

- NUnit3TestAdapter:
  Visual Studio needs it for LUT to find the tests

- Microsoft.NET.Test.Sdk:
  LUT asked if it was installed thus assuming it is required
  
It was also necessary to explicitly set the startup object of the test
project to MoreLinq.Test.Program because of additional possible start
points created by one or other of the NuGet packages above.

FullGroupJoin TSecond type param doc typo; fixes morelinq#349

List From in README & package description

Add FullGroupJoin to release notes

2.5 version stamp

Note about maintenance release

Pin projects to C# version 7

Add Partition; closes morelinq#210

Partition splits a sequence in two or more generally groupings

.NET 4.0 can be targeted with System.ValueTuple