Version v2.51 (April 2018)

Converted from fortran 77 to fortran 90.

Adopted allocated temporary arrays in many routines to save memory.

Changed atomic database from simple arrays to fortran 90 structures. No longer passed as arguments to most routines, now contained as derived data types in a module.

Changed procedure for passing lte level populations in to fstepr2, rstepr2, the routines responsible for writing and reading rrcs.