Refactor: Default constructor usage and pressure conversion for unit system

Special units required for particular quantities, for example:

  type(unit_t),        public :: unit_one           !< For unitless quantities and arithmetics with units.
  type(unit_t),        public :: unit_angstrom      !< For XYZ files.

are defined in the initialiser of unit_system_init. This is unnecessary, and should be replaced by declarations that use the default constructors to initialise. Furthermore, these could be declared with the parameter keyword.

The unit system should be extended to included conversion of pressure. See stress.F90 for the conversion factor, real(real64), parameter :: au_to_GPa, which should be migrated into unit_system.F90

Parameterised derived types might be of interest