Remove big Ms from MGA formulations

[gnawin]

See below description from the link (https://spine-tools.github.io/SpineOpt.jl/latest/advanced_concepts/mga/#How-to-set-up-an-MGA-problem):

The original MGA formulation is non-convex (maximization problem of an absolute function), but has been linearized through big M method. For this purpose, one should always make sure that units_invested_big_m_mga, connections_invested_big_m_mga, or storages_invested_big_m_mga, respectively, is sufficently large to always be larger the the maximum possible difference per MGA iteration. (Typically the number of candidates could suffice.)