v0.7.4

DispersiveShallowWater v0.7.4

Diff since v0.7.3

Merged pull requests:

• Added kdv_1d non dimensional solitary wave example (#217) (@cwittens)
• Throwing error when PeriodicDerivativeOperator is not matching boundary condition (#218) (@cwittens)
• Try to fix downgrade (#219) (@JoshuaLampert)