Add todo item

Author: mdaeron

todo.md

@@ -4,4 +4,5 @@
 * How to deal with `D4xdata.standardize()` having different effects depending on `method`?
 * Turn `D4xdata.R17_VPDB` (and perhaps also `D4xdata.R18_VPDB`) into a property, so that there is no risk of redefining `D4xdata.R17_VSMOW` without updating `D4xdata.R17_VPDB`; In this case, the setter methods for `D4xdata.R17_VPDB` and `D4xdata.R18_VPDB` should raise an exception.
 * improve `test_virtual_data()` to populate with non-default parameters
-* use a true CSV parser
+* use a true CSV parser
+* add (many) plot customization options to CLI