Changes to As<>() function in Format and a new Is<>() function

[ardasener]

This PR should resolved #130 and #131. It makes the following changes/additions:

• A new templated Is<>() function that can check whether the Format pointer is of a particular concrete Format type.

if(format->Is<CSR<int,int,int>>()){
  // Do something
}

• Some tests are also added for Is<>().

• Both in the new Is<>() function and the old As<>() function, std::remove_pointer is used to handle calls with pointers correctly as well. This is done to avoid a common misunderstanding (where the user calls the function with the pointer type, not the base type) and also done in some other libraries we reviewed. Unlike what I suggested in Decay in As<>() #131, std::decay is not used as it actually works differently then I thought.