Define variables through new @defvar macro

docs/src/sequence_optimisation.md

@@ -5,16 +5,69 @@ In most MR sequence building software, the user will have to set all of these fr
 
 In MRIBuilder the internal free parameters are not set directly. 
 Instead, they are inferred using a non-linear, constrained optimisation.
-For each sequence type, the developer defines how to compute various summary variables from the `BuildingBlock` free parameters, such as [`echo_time`](@ref), [`repetition_time`](@ref), [`resolution`](@ref), [`gradient_strength`](@ref), [`diffusion_time`](@ref), etc.
+For each sequence type, the developer defines how to compute various summary variables from the `BuildingBlock` free parameters, such as [`variables.echo_time`](@ref), [`variables.duration`](@ref), [`variables.resolution`](@ref), [`variables.gradient_strength`](@ref), [`variables.diffusion_time`](@ref), [`variables.duration_transverse`](@ref) etc.
 A user can then create a specific instantiation of the sequence by fixing any of these summary variables to their desired values (or setting them to `:min`/`:max` to minimise/maximise them).
 In addition to the user-defined constraints, this optimisation will also take into account any [scanner-defined constraints](@ref scanners).
 Internally, MRIBuilder will then optimise the `BuildingBlock` free parameters to match any user-defined constraints and/or objectives.
 This optimisation uses the [Ipopt](https://github.com/coin-or/Ipopt) optimiser accessed through the [JuMP.jl](https://jump.dev/JuMP.jl/stable/) library.
 
 ## [Summary variables](@id variables)
+All variables are available as members of the [`variables`](@ref) structure.
 ```@meta
 CollapsedDocStrings = true
 ```
+```@docs
+variables
+```
+```@docs; canonical=false
+variables.N_left                    
+variables.N_right                   
+variables.TE                        
+variables.TR                        
+variables.all_gradient_strengths
+variables.amplitude                 
+variables.area_under_curve          
+variables.bandwidth                 
+variables.bmat                      
+variables.bmat_gradient
+variables.bval                      
+variables.delay                     
+variables.diffusion_time            
+variables.duration                  
+variables.duration_dephase
+variables.duration_state            
+variables.duration_transverse       
+variables.dwell_time
+variables.echo_time                 
+variables.effective_time            
+variables.flat_time                 
+variables.flip_angle
+variables.fov                       
+variables.frequency                 
+variables.gradient_strength         
+variables.gradient_strength_norm    
+variables.lobe_duration             
+variables.net_dephasing             
+variables.nsamples
+variables.oversample                
+variables.phase                     
+variables.qval                      
+variables.qvec
+variables.ramp_overlap              
+variables.readout_times             
+variables.resolution                
+variables.rise_time                 
+variables.slew_rate
+variables.slew_rate_norm            
+variables.slice_thickness           
+variables.spoiler_scale             
+variables.time_to_center            
+variables.voxel_size                
+variables.Δ                         
+variables.δ
+```
+
+## Variables interface
 ```@autodocs
 Modules = [
     MRIBuilder.Variables,