Remove requirement for R_HOME environment variable to be set

[AndrewIOM]

Is your feature request related to a problem? Please describe.
Having to set the R_HOME environment variable is an extra step that adds complexity for new users. Although RProvider is configured to detect this and try to set it automatically, and this works for compile-time, the error 'R Home not set' appears at runtime.

Describe the solution you'd like
Investigate the reason that this error still appears - despite RProvider passing R Home to RDotNet - and add code to address this shortcoming.

Describe alternatives you've considered
Clearer documentation could be provided to help set the R_HOME value permanently on a platform-specific basis, but this does not fully address the issue.

Additional context
NA