Update the documentation (formulation and tutorial/how-to) with the use of a tighter formulation for storage assets

[datejada]

Update documentation on early work from Maaike's research to improve energy storage constraints. See attached PDF

SpineOpt-formulation-GM.pdf

[datejada]

Some initial questions to address with @g-moralesespana:

• What if we don't have a limit $C, D$ for the investment variables? -> Add a validation, if there is no investment limit, then the user can't use the binary method.
• We use minimum/maximum profiles instead of $\phi$. So, how does equation $(108)$ change? -> it doesn't change.
• Can we have only one formulation for with and without storage inflows? -> We won't add the equations with storage.

[datejada]

After review with @g-moralesespana we will only implement the following constraints in the model:
(102), (103), (107), (109), (110), (111), (112), (113), (114).