C2INAV — navigation_covariance_type should have a max length

To allow application to statically size (bound) their data structures variably sized datatypes should have a tag to bound the size.
Accordingly navigation_covariance_type should have a Length tag.
The maximum value = 253
(sum(3n+1; n=7) = 22 * 23 / 2) (as there are seven topics each with three values - plus time that can contribute to the covariance; this is very much a worst-case upper bound)

Add Length Tag to covariance value attribute

Add Length tag value = 253 as per issue description

State that sequences are bounded in the IDL PSM