Release 0.6.3

Major changes to STPSingleHistogramFilter

Move default declared destroctor for STPSingleHistogramFilter into source code so compiler won't complain about using incomplete type of Pimpl.
Remove constructor of STPSingleHistogramFilter to predict the size of the texture because it won't make any difference to the performance. Now STPSingleHistogramFilter is trivally constructable.
Augment the data structure, now the output histogram is no longer bound to a particular STPSingleHistogramFilter instance, but rather, user can ask for a single histogram buffer where the output will then be stored.
Split STPSingleHistogramFilter::STPHistogramBuffer with two versions, being default-allocated and page-locked-allocated. The buffer passed to external will always be using page-locked allocator for fast device memory transfer.

Refactor code base using new features in C++17 like conditional initialiser and structured binding,
Enable move constructor/operator in STPChunk and STPChunkStorage.
Simplify class construction using initialiser list, for STPRainDrop, STPSingleHistogramFilter and STPFreeSlipGenerator and various STPEnvironment classes.
STPChunk will now always allocate memory for internal texture during construction.
Fix some typos.
Fix a detrimental bug that causes host memory to be freed before CUDA stream has finished execution. To resolve that we use CUDA stream host callback.
Augment STPPinnedMemoryPool such that it can be used on pagable memory. To avoid confusion it has been renamed to STPMemoryPool and use template parameter to denote the memory type of it.

Partial resolution to issue #21

Use remove_pointer to determine the base type for cudaStream_t instead of hard coding.
Change the storage type in STPChunkStorage from unique_ptr<STPChunk> to STPChunk.
Simplify chained function call to set compiler and linker flag in STPDiversityGeneratorRTC.