this change utilizes some of the features from library "frozen", which
was included upstream for the grid profile parser. to improve code
maintainability, a couple of std::maps mapping strings to values or the
other way around were introduced in OpenDTU-OnBattery-specific code at
the expense of some flash and computing overhead.
library "frozen" offers constexpr versions of map and string, which
saves initialization code and offers slightly faster lookups. this
brings the binary size down by ~25kB and should provide a small
performance improvement at runtime.
