Update MM tag regex to include upper case "Ambiguity codes"

 The regex for the MM tag (section 1.7):
```
  MM:Z:([ACGTUN][-+]([a-z]+|[0-9]+)[.?]?(,[0-9]+)*;)*
```
  specifies `([a-z]+|[0-9]+)` for the modification codes which is lower case only but this does not therefore parse the ambiguity codes like "C - Ambiguity code; any C mod"


the spec may want to update it to
```
([a-z]+|[A-Z]|[0-9]+)
```