Durations not parsed correctly

[davidbau]

The parser doesn't support durations ending with a slash, and it doesn't parse mixed-fraction durations correctly. For example,

G/ parses and G3/2 parses, but G3/ does not.

Here is an example from the ABC website that I'm trying to parse, where this appears a lot:

X:2
T:8th Sonata for piano
%%staves {1 2}
C:L. van Beethoven
M:C
L:1/16
Q:1/8=66
F:http://richardrobinson.tunebook.org.uk/tune/6525
K:Cm
% .. even when there are a lot of notes
V:1
!fp![E,4G,4C4]- [E,3/G,3/C3/]!3![G,/C/]!4![G,3/=B,3/D3/]!5![G,/C/E/] ([=A,4C4E4]!4![=B,2D2])z2|
!fp!!3![=B,4D4F4]- [B,3/D3/F3/][B,/D/F/][B,3/D3/G3/][B,/D/A/] ([B,4D4A4]!3![C2E2G2])z2|
V:2
[C,,4E,,4G,,4C,4]- [C,,3/E,,3/G,,3/C,3/]!2!E,/!3!D,3/!4!C,/ (!2!^F,4G,2)z _A,,|
_A,4-A,3/!2!A,/!1!G,3/=F,/ E,4-E,2z2|

Related to this, durations with mixed fractions aren't parsed correctly:

f11/4 should be read as "1 and 1/4", i.e., equivalent to f5/4, but instead it is read as "eleven quarters".

This type of thing appears a lot in ABC found online, for example, in http://www.thefatlute.eu/rapid/view_song/g/1122/407

In my ABC sequencer, this is what I am using to parse durations - not sure it's 100% correct, but it seems to work in practice with music people have posted. https://github.com/PencilCode/musical.js/blob/master/musical.js#L1282