module enhancements part 1: knot and coeffs boundary parameters

[CeresBarros]

as per PR#5:

• allow supplying several knot limit values per variable: upper and lower knot limits are supplied as a named list with names being a variable, but only one value is allowed per variable (so if a given variable is used for different pw components, the same upper/lower limit values will be applied). Ideally we should be able to supply different boundaries for each pw component/coefficient

• also supply coefficient lower/upper boundary values as named list: like for knots. Instead of applying the same values to all variable coeffs.

From further detail here