Minor fixes in read_nodelist, read_edgelist and as_network methods

[BBieri]

Description

Import and export

• Fixed read_nodelist #172 by removing redundant header argument in read_nodelist() and
  read_edgelist()

Package

• Fixed as_network #173 by extending as_network() method to convert correctly form
  an {igraph} to a {network} object.
• Removed ggraphgrid() documentation

Checklist:

• PR form
  • This pull request has an informative title
  • Description above itemizes changes under subtitles, e.g. "## Data""
  • Any closed, fixed, or related issues are referenced and explained in the description above, e.g. "Fixed #0 by adding A"
  • Package builds on my OS without issues
• PR checks all pass for latest commit
  • CodeFactor check: Package improves or maintains good style
  • Package builds on Mac
  • Package builds on Windows
  • Package builds on Linux
  • CodeCov check: Package improves or maintains good test coverage
• Documentation
  • Any new or modified functions or data have roxygen style documentation in their .R scripts
  • Any longer functions are commented inline so that it is easier to debug in the future
  • PR description above and the NEWS.md file are aligned
  • DESCRIPTION file version is bumped by the appropriate increment (major, minor, patch)