Dose Area Product with zero value is handled incorrectly

[Luuk]

The value of the DAP is not shown for a rdsr with an event having a DAP value of 0 (and a very small reference dose) Also the skin dose mapping fails on this zero value. Reason is that

if dose_area_product:

is used and a float with zero value will evaluate to false

[Luuk]

Refs #565 Dose Area Product with zero value is handled incorrectly

Changed 'if dap:' statements to try-except statements to handle zero values correctly.

→ <<cset 5152cb1f6925>>

[Ed McDonagh]

Copying CHANGES into docs/changes.rst. Refs #565

→ <<cset d82460f27556>>

[Ed McDonagh]

Merged in Issue565DAPIsZeroHandledIncorrectly (pull request #133)

Fixes #565 Dose Area Product with zero value is handled incorrectly

Thanks @LuukO

→ <<cset 095de7aa2b88>>