Fix grammar to accept numbers without leading zeros, e.g. ".45"

[hpodhaisky]

Many matrices of the matrix market are being parsed incorrectly because of the missing leading zero.

Example: https://sparse.tamu.edu/Grund/b1_ss

%%MatrixMarket matrix coordinate real general
%-------------------------------------------------------------------------------
% UF Sparse Matrix Collection, Tim Davis
% http://www.cise.ufl.edu/research/sparse/matrices/Grund/b1_ss
% name: Grund/b1_ss
% [Unsymmetric Matrix b1_ss, F. Grund, Dec 1994.]
% id: 449
% date: 1997
% author: F. Grund
% ed: F. Grund
% fields: title A b name id date author ed kind
% kind: chemical process simulation problem
%-------------------------------------------------------------------------------
7 7 15
5 1 -.03599942
6 1 -.0176371
7 1 -.007721779
1 2 1
2 2 -1
1 3 1
3 3 -1
1 4 1
4 4 -1
2 5 .45
5 5 1
3 6 .1
6 6 1
4 7 .45
7 7 1

And the .45 and the .1 are not being read.

Note that ../../nalgebra-sparse/src/io/matrix_market.pest is correct.

Also: ".45".parse::<f64>() works.`

[alexhroom]

thanks!