Ability to configure BaseTableMaxThreads at the Data Group level

[ni-rwhite]

Add the ability to adjust the BaseTableMaxThreads at the Data Group level. This will allow for "right sizing" memory allocation that is consumed during the build process. 

For example, I have low priority elasticubes residing in the default data group. High priority elasticubes reside in a "High Priority" data group with custom settings for "Stop When Idle", RAM, etc. Currently, our environment is configured to load 3 base tables concurrently (BaseTableMaxThreads  = 3). This setting impacts both high and low priority data groups. I want to be able to set my low priority data group to only load one table at a time (BaseTableMaxThreads  = 1) to reduce memory consumption on the server during builds. I want my high priority data group to continue loading 3 tables at a time. The trade off is the default data group will load data slower but will consume less memory/CPU during builds, freeing up resources for the high priority data group.