mts: use stream IDs defined in package codecutils for MTS encoding

[Trek Hopton]

We should use the stream IDs in the package codecutils in helpers.go when encoding mts. Currently there are two sets of stream IDs which is less maintainable, the second which is in package mts should be removed.

[Trek Hopton]

I'm now thinking of moving the SID list and the SIDToMIMEType function from codecutils to package pes since the SIDs are specific to PES data.

[Trek Hopton]

Resolved in PR #440