Add Pkgconfig files so other apps can use pmtf

[mormj]

Also, rename a bunch of the pmt/ to pmtf/ - still need to settle on a name, but wanted to keep it consistent to make the pkgconfig and cmake searches easier and only key off one name (pmtf)

[mormj]

@jsallay - am I doing this right? made a get_pmt_string method so that the pmt_wraps could be interpreted as string

Wondering if that set of APIs should be restructured rather than global methods.

[jsallay]

That is fine for now. In the future, I would like a template function, but need to think it through because it works well for some types and is more clunky for others. For example,

pmtf::get<std::string>(x);
pmtf::get<uint64_t>(x);
// but not
pmtf::get<vector<float>>(x);  // This doesn't make sense because you are getting a pmt, not a vector.