XTCE12 — Variable length strings need optional maximum length designator

way to specify the maximum length for variable length strings is missing. A related issue is for pascal-style strings is the bit/byte order of the preceding length is missing. (that makes this another issue but I am tired of adding issues to this thing!) Note: this is actually an old issue which somehow never made it into the issues list here that I can fine. I believe the original issue was relayed to me by EADS 5+ yrs ago. I thought they intended to put it in.

Working string issue in XTCE12-110

Propose to address this issue along with the additional string data encoding concerns in XTCE12-110.