v0.15.4

ParametricOptInterface v0.15.4

Diff since v0.15.3

• allow model with only parameters to be solved

Merged pull requests:

• Allow solver with no decision variables (only parameters). (#241) (@joaquimg)
• Fix SCS test tolerances in test_JuMP.jl (#243) (@joaquimg)
• Prep for v0.15.4 (#244) (@joaquimg)