An error floor for 1D advection

[Balavarun5]

Commit : 5a0b743

Issue

The L1 norm of an advected 1D wave has an error floor of 1e-8 as shown in the image below.

After a certain N_LGL, the error doesn't decay as N_LGL ** (- N_LGL)

This error could be a result of the Issue discussed in #27 where the volume integral term has a non trivial error. The plot of L1 norm of volume integral term vs N_LGL at the time of raising this issue is shown below