api: allow lambda tables to be user-specified via a text file

This change allows easy experimentation with the lambda tables. One can
cut-paste the existing tables from TComRom.cpp into a text file and hash(#)
comment the C constructs (variable names and braces) and arrive at a functional
lambda file, then edit to taste.
simplify: getLumaIntraDir()[x] -> getLumaIntraDir(x)
lambda-file: detect and report if lamda file containts too many values

It seems likely that this is an unintended error the user would want to be
made aware of.
lamba-file: make improper number of constants a fatal error

Having an invalid set of lambdas is likelty to be completely useless