switch trunc() to the new way (FunctionExpression -> AbstractExprCall)
added explicit test for trunc()
moved both argument checking and the kwargs to args conversion to AbstractExprCall (from NumpyFunction), but bypass it in GenericExprCall (because we do not have the information there)
added (commented) tests for the argument-checking code
added missing CRLF at end of files
initialize ProcessGroup.versions in __init__
transformed NumpyFunction.func_name into a *class* property, so that we can use it even when no instance exists yet
update TODO, fix typo
* FillArgSpecMeta now ignore the first argument from _compute methods (ie strip "self")
* check that we have enough args in addition of checking that we do not have too many
* added context as first argument of _compute
* _eval_args does not evaluate any arg whose name is in cls.no_eval
fixed _eval_args for kwargs
convert ValueForPeriod and Lag to AbstractExprCall + no_eval