Sam: add tests to the public API

**Added the following tests**:

* unclipped start and stop
* copy constructor tests
* copy assignment test
* tags of types char, int and double
* char_tag when tag is not a char
* string_tag when tag is not a string
* is_missing support for tags

major strides towards addressing issue #4
Not related to this pull request, but I really hate the name of this header (is_missing.h). Let's create a gamgee.h header that includes this, sam.h, variant.h, etc., so users only need one header in most cases, and don't have to type this embarrassing name