Remove unnecessary include directives in header (.h) files

[adisidev]

We should study and implement the best practices as to what and what not to include in source files compared to header files. The conventions seems to be too include as few include directives as possible. I have noticed inconsistencies in our code; the convention followed seems to be ambiguous.

These may assist in further reading and rewriting:
https://stackoverflow.com/questions/2596449/including-includes-in-header-file-vs-source-file
https://softwareengineering.stackexchange.com/questions/167723/what-should-and-what-shouldnt-be-in-a-header-file

[adisidev]

Circling back to this; removing various headers does not seem to affect compilation. Thus, some files may be including header files by virtue of being loaded after a file that uses a particular header. However, we should not let this order matter.