Altitude field validation

[japonicus]

Altitude is not validated at the point of entry.

Altitude values such as 'low' or '10m' are accepted. At the point of final submission the whole set is rejected with an uninformative message: 'The data could not be saved'.

Returning to the record list does expose validation warnings on the altitude field. These warnings are not cleared even after the altitude field is either blanked or set to a valid value.

See #3 for umbrella issue.

[sacrevert]

hi @johnvanbreda would you be able to focus on this next?

[johnvanbreda]

Now done.