Make Resource Constructor Explicit

[kab163]

Currently, the resource test may be creating temporary Resource objects for certain tests instead of doing what we expect - so making the Resource constructor explicit.

However, this will require a change to the Reassignment test (and any user code which does a reassignment).

[kab163]

Hoping this PR can fix a couple of issues:

• First, once the Resource constructor is explicit, some of the other operator overload functions could be added back in - see here. They had to be removed because the Host2 dummy struct in the resource test makes the context model initialize itself and confused the MSVC and SYCL compilers since it did not know how to resolve which operator overload to use for Host2 and Host in the following get and try_get calls.
• Second, we are hoping that this will also resolve an issue in Umpire where the compiler does not know whether it should call allocate(Resource r, size_t bytes) or allocate("my_string", size_t bytes) since a char* can be converted to anything.