Use better int type than long in C++ code

[tovrstra]

This is much clearer and is suggested by cpplint. This touches a lot of code, so it is best to wait till 2.2.0.

[sfias]

what would happen than is one tries to run HORTON on a 32-bit system?

[tovrstra]

It would work, not super efficiently. Should we care?

[tovrstra]

@matt-chan We should rethink this. This is the detailed explanation and we interpreted too narrowly:

https://google.github.io/styleguide/cppguide.html#Integer_Types

It is probably better to ignore this problem until we have better idea how to handle this.

[tovrstra]

For now, the corresponding check is disabled in tools/qa/trapdoor_cpplint.py. We can switch it back on after we decided how to deal with this.