T is not a recognized residue type for DNA

[GiovanniBussi]

Currently, we recognize in MOLINFO the following DNA residue types:

DX
DX5
DX3
DXN

with X = A, T, C, or G.
On the other hand, for RNA we recognize:

RX
RX5
RX3
RXN
X
X5
X3
XN

with X = A, T, C, or G.

As a consequence, if a residue is named T one cannot use special selections (it should be renamed as DT).

I think we never need to know if a residue is RNA or DNA type, since in order to find symbols we just check that allowedResidue("rna",resname) || allowedResidue("dna",resname).
Probably it would be harmless to add X, X5, X3, and XN to DNA as well.