fix GRASS v.net modules (fix #19904) [processing]

[alexbruy]

Description

There are several issues with GRASS v.net modules in Processing:

• points does not snapped to the network by default
• v.net failed to produce output even if points snapped to the network
• stdout handling is broken in all GRASS algorithms
• broken v.net.report and v.net.nreport modules

This PR addresses these issues. Fixes https://issues.qgis.org/issues/19904 and https://issues.qgis.org/issues/21142

Checklist

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[PedroVenancio]

Hi @alexbruy ,

I also found this issue in v.net.salesman https://issues.qgis.org/issues/21142

Do you think it can be 'easily' fixed and added to this PR?

[alexbruy]

@PedroVenancio I will look into it.
Regarding v.net.report and v.net.nreport looks like they are removed from GRASS, I can't find them in the GRASS docs

[PedroVenancio]

Thank you very much @alexbruy !

Regarding v.net.report and v.net.nreport looks like they are removed from GRASS, I can't find them in the GRASS docs

They are part of v.net as different operations
https://grass.osgeo.org/grass76/manuals/v.net.html

But I've never used them, I don't know much about them.

[PedroVenancio]

Thank you very much @alexbruy !!