`createSSURGO()` should create indices for tables/columns commonly used by soilDB queries about soildb

[brownag]

This (adding indices) is done in b97d4c8

I think creating template databases, and going down the path of creating the custom table creation statements, is a different, more involved issue that is possibly out of scope for soilDB. It represents a fundamental difference in design between SSURGO Portal and createSSURGO(). A benefit of current createSSURGO() is we are not locked in to any specific schema nor need for file--based templates.

[dylanbeaudette]

Good call:

• foreign keys are only really useful if you need DELETE CASCADE functionality (more in-line with SSURGO portal)
• "optimal" indexing beyond the standard (mukey, cokey, chkey, etc.) is an open-ended problem
• indexing before insert is a performance hit
• over-indexing to cover all possible cases is storage concern

I do think that a short tutorial on building a custom DB from createSSURGO() is worth the effort and I'm happy to help.