Updated and added new functions related to headers, resolved ODR issues, resolved clang-tidy warnings (#50)

* Bugfix/odr violations (#47)

* Make common non-member functions inline, remove unreachable line from get_line_buffer

* [skip ci] Fix namespace comments

* Resolve clang-tidy warnings (#48)

* Resolve clang-tidy warnings, update single_header_generator.py

* Update single header test, resolve additional clang-tidy warnings

* Add header and raw_header methods,  update header usage methods error handling, write new and update existing unit tests

* Update parser error messages, fix parser tests

* Add [[nodiscard]] where fitting, update unit tests (#49)

* Add const where fitting, make splitter class members private, add #pragma once to ssp.hpp

* Modify header parsing for empty headers, update old and add new tests for header parsing

* Enable the parser to accept a header with one empty field, update unit tests

* Fix test CMakeLists.txt typo

test/test_single_header_main.txt

@@ -0,0 +1,12 @@
+int main() {
+    using quote = ss::quote<'"'>;
+    using escape = ss::escape<'\\'>;
+    using trim = ss::trim<' '>;
+
+    std::string data = "1,string,2.34,c";
+
+    ss::parser<quote, escape, trim, ss::multiline> p{data.c_str(), data.size()};
+    auto tup = p.get_next<int, std::string, float, std::optional<char>>();
+
+    return 0;
+}