Int64 support

[icosaeder]

1. Support of 64-bit integers to handle blocks of more than 4 Gb per computing node.
2. Support of using the library in C++ projects.
3. Fixes of minor warnings regarding type conversion and unused/uninitialized variables.

[icosaeder]

I found this library super efficient and very well-scalable, but the problem was the crashing when the size of the block per computing node was bigger than 4 Gb. My changes fix this problem by introducing the support of 64-bit integers to express indices and sizes. To switch to 64-bit integers, the library should now be compiled with '-DPMRRR_INT64' flag, which I mentioned in README.