Support for IOP 4.3 clauses 3.10.2 and 4.6

[tafinho]

On the current validator, the following validation error is obtained if a @type = static and period@start is non zero:

• If MPD is of type "static" and the first period has a start attribute the start attribute shall be zero.

This directly contradicts the following IOP 4.3 clauses:

3.10.2

where @type must be static and period@start must be present and relative to MPD's AST.

4.6.2

The MPD@availabilityStartTime should be removed or be maintained from the live MPD since all resources referenced in the MPD are available. If the MPD@availabilityStartTime is maintained for a portion of the live program that is offered in the static MPD the Period@start value (including the presentation time offset and the start number) and the presentation duration shall be set accordingly. The relationship to the wall-clock time should be maintained by offsetting the Period@start without changing the MPD@availabilityStartTime.

[RufaelDev]

yes this is also common in case of live multi-period content converted to a static for example were ealier presentation would be missing. This case of live to static conversion is what introduces this, i think the main restriction would be the profile, in the on demand profile @start would be 0, but in case of static mpd with live profile this may not be the case, such as when it resulted from a live to static conversion

[Phencys]

Closing as this is now solved