Implement macros for all functions (where appropriate)

[stelmo]

Let's make macros to really make the user interface clean. I have implemented fba macros in #53 (see the last few commits) and @marvinvanaalst has implemented a macro for reaction adding in #59 . @exaexa and I were also talking about this extensively on slack, it would be really cool to have something like this (from slack @exaexa):

vs = @variants
  knockout(123)
  knockout(4345)
  ...
end

@mod_variants! vs
  remove_reaction(123)
end

@combine_variants! vs
  no_modification()
  add_random_ATP()
  add_some_toxin()
end

Currently I have an fba mini version of this working as shown below:

using COBREXA
using Tulip

model = read_model(joinpath("e_coli_core.json"))
biomass = findfirst(model.reactions, "BIOMASS_Ecoli_core_w_GAM")
glucose = findfirst(model.reactions, "EX_glc__D_e")

vec = @flux_balance_analysis_vec model Tulip.Optimizer begin
    modify_objective(biomass)
    modify_constraint(glucose, -8.0, -8.0)
end

[exaexa]

anyway, it might be better not to over-macro the code in some parts. A minilanguage is certainly viable for combining the modifications (although there should be a normal functional variant of the code), but the example below may read easier as a normal function:

res = flux_balance_analysis_vec(model, Tulip.optimizer,
  modifications=@mods begin
    objective(biomass)
    constraint(glucose, -8, -8)
  end)

(even then the code can still be shorter with a normal vector [])