Examine the electron density retrieval settings

[Ian Culverwell]

#700 describes the implementation of a tool to carry out variational retrievals of electron density using differenced bending angles. This was included in ROPP-11. Some sensitivities remain, such as the choice of minimum acceptable values of control variables during the 1dvar iteration. Initially getting this wrong caused problems during testing, which were resolved in r6868. But the whole question deserves more considered thought and experimentation, and this is ticket is the place to record such deliberations.