Added script to generate concepts for postgres DB.

[stefanhgm]

Hi Alistair,

I've created a routine to build concepts for PostgreSQL based on the existing mimic-iii code. Not sure if the code quality is sufficient for you, but nevertheless I thought it might be useful for others.

• concepts/postgres-functions.sql: identical to mimic-iii mimic-code/concepts/postgres-functions.sql except the schema was set to 'public'
• concepts/postgres_make_concepts.sh: used loop structure from concepts/make_concepts.sh and added some regexps
• concepts/Readme.md: documented the usage (copied from mimic-iii) and known problems

Let me know if there is something I should improve.