Remove requirement for R_HOME environment variable to be set

**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

Start by tracing how RProvider detects and passes R Home to RDotNet, then reproduce the runtime error "R Home not set" without manually setting R_HOME. Done means RProvider can initialize R at runtime without requiring users to set R_HOME themselves.

Written by the indexing model from the issue text.