Implement total() aggregation function

[benclmnt]

• Returns 0.0 when called on non integer / non float columns
• Always returns floating point number
• fix: default for sum() should be NULL when there is no non-NULL row per docs

related: #93

[penberg]

Hey @Ramkarthik, does this PR fix https://github.com/penberg/limbo/issues/118?

[benclmnt]

@penberg probably not, will need another PR to address 118 specifically

[Ramkarthik]

Hey @benclmnt, please let me know if you're working on #118. If not, I will continue working on it.

[benclmnt]

@Ramkarthik I've created PR #119 . Could you help to review when you have time?