Performance - omit 1.0000 when writing out LP file

[willu47]

linopy LP files are created with lots of extra characters when the coefficients are 1:

c2:
-1.000000 x6
+1.000000 x8
=
0.000000

These can be omitted:

c2:
- x6 + x8 = 0.00000

[FabianHofmann]

Sound reasonable! Do you happen to know whether that is a general format, i.e. can we expect all solvers to be able to read this?

[willu47]

I've tested with Gurobi, GLPK and CBC and those seem fine. I don't have access to other solvers at the moment.

[FabianHofmann]

closed by #114