Replace using namespace in headers with explicit namespaces.

[carlosperate]

This PR is one part to resolve issue lancaster-university/codal-microbit-v2#240:

• using namespace codal shouldn't be used in header files codal-microbit-v2#240

To avoid namespace pollution headers should not use using namespace codal;, and instead it can be added into any .cpp files that needed (all of them in this repo).

I still need to test this PR in isolation with a normal CODAL build, so I'll leave this as a draft PR until I've finished that.

[carlosperate]

Okay, had to leave the using namespace codal in the NRF52PWM.h header file, as some .cpp files in codal-microbit-v2 depended on it. Once we fixed all the namespacing in codal-microbit-v2 we can remove it from NRF52PWM.h in a different PR.

I think this approach, keeping some of the namespace in this PR so that is safe to merge, even if it means more PRs in total, is probably a bit easier to manage than orchestrating several PRs to be merged in order.