Add extension error if Plots is not loaded

[ocots]

Add extension error if Plots.jl is not loaded. For the moment we have

julia> using CTBase
julia> sol = ...
julia> plt = plot(sol)
ERROR: UndefVarError: `plot` not defined
Stacktrace:
 [1] top-level scope
   @ REPL[46]:1

[ocots]

The last thing I had which is not working is the following:

julia> using CTBase

julia> plot(sol)
ERROR: ExtensionError. Please make: julia> using Plots
Stacktrace:
 [1] plot(::OptimalControlSolution; kwargs::@Kwargs{})
   @ CTBase ~/Research/logiciels/dev/control-toolbox/CTBase/src/CTBase.jl:219
 [2] top-level scope
   @ REPL[4]:1

julia> using Plots
[ Info: Precompiling CTBasePlots [0ad68a25-3dcc-5afc-86be-7bb96764f193]
WARNING: using Plots.plot in module Main conflicts with an existing identifier.

julia> plot(sol)
ERROR: ExtensionError. Please make: julia> using Plots
Stacktrace:
 [1] plot(::OptimalControlSolution; kwargs::@Kwargs{})
   @ CTBase ~/Research/logiciels/dev/control-toolbox/CTBase/src/CTBase.jl:219
 [2] top-level scope
   @ REPL[6]:1

[jbcaillau]

@ocots OK, but no one is supposed to use CTBase then solve / plot...

[ocots]

@ocots OK, but no one is supposed to use CTBase then solve / plot...

Yes but it is the same from OptimalControl.

For the moment, after using OptimalControl there is an error that we raise if we call solve without using NLPModelsIpopt, an error that we raise if we call Flow without using OrdinaryDiffEq but if we call plot there is an error from Julia, not from us. It is not consistent.

[jbcaillau]

Ah, sure.