nexml export: letters in seq element of standard datatype

[ghost]

Several studies emit letters in the seq element for standard datatypes. As mentioned on https://sourceforge.net/mailarchive/message.php?msg_id=31802970
it is not clear if this should be allowed in NeXML.
Affected studies: S10004 S10039 S10051 S10090 S10092 S10137 S10153 S1016 S10186 S1018 S10191 S10192 S10208 S10243 S10272 S10279 S1051 S10549 S10601 S10867 S10905 S10917 S11167 S111 S1127 S1135 S1138 S1158 S1166 S116 S1174 S118 S1193 S11957 S120 S12298 S12319 S1238 S12681 S12816 S12890 S12904 S12910 S12922 S12925 S13149 S13223 S1322 S132 S13353 S13606 S1371 S1392 S1411 S1419 S1423 S1424 S1432 S1446 S1510 S1513 S1541 S154 S1556 S1570 S1611 S1624 S1640 S1666 S1681 S1697 S1705 S1739 S184 S1851 S1856 S1861 S1892 S1911 S193 S1942 S1958 S1996 S200 S2055 S2078 S2086 S2122 S2143 S2146 S2186 S2188 S2212 S226 S229 S231 S2372 S238 S274 S289 S299 S304 S307 S331 S332 S333 S336 S357 S363 S388 S395 S404 S421 S575 S592 S644 S647 S689 S695 S715 S725 S749 S791 S796 S828 S834 S85 S867 S883 S884 S893 S912 S913 S914 S958 S985 S988 S9939 S9949 S9953 S9974 S9990 S9998

Reported by: mholder