Reinstate PCDB query methods

#### Description:

Could we please reinstate and expand on the PCDB query methods? These were originally defined and created in issue #35 and PR #39

I have a (non-urgent) project related need to interrogate the database but looks like we have removed the methods in the tidy-up from issue #121 / PR #124

Once the previous methods are reinstated, would be good to add the following functionality:

- Ability for users to define which version of the database to query as there are two relevant ones:
https://www.ncm-pcdb.org.uk/pcdb/pcdb10.dat
http://www.boilers.org.uk/data1/pcdf2012.dat
- Query values in Tables 181 (installation costs), 191 (current fuel prices) & 199 (Historical fuel prices). Note that there are multiple versions of Table 199, I have no idea how to handle that!!!