Harmonise spelling of input variables for initial fields

[edoddridge]

The variables initHfile, initUfile, and initVfile all have a lower case "f", while initEtaFile uses an upper case "F". These variables should use a single standard, and the rest of the code should be checked for compliance with this choice.

This inconsistency is related to #40, which will require a choice of CamelHumping or snake_case for variables and subroutines, and then a thorough sweep of the code to implement the choice.

I'm leaning towards "f" and snake_case for these variables, as discussed in #40.