Try to identify testthat bugs

Add maptt proxy

See merge request !2

We used to return the MapTT factory objects directly, preventing objects
to be updated. Thus, after initialization, `maptt` was still null, and
we were unable to retrieve maptt through the Proxy.

To fix this behaviour, we now use functions to return objects.

version 0.0.6

MapTT

See merge request !1

R timestamps are in seconds, whereas Javascript has them in ms, so
we convert them.

If times are stored as characters but contain only digits, we convert
them to numeric and process them as such.

tt.event's time column represent timestamps but are stored as characters (!)

Changing origin day for julian days has makes these dates relatives, and useless for tests.
We now use only absolute dates, except in the specific julian days test.

Also, setting timestamps directly inside the time column has obliged us to modify dates
in some tests.

Convert times from several types to timestamps once and for all.
Times are also stored as timestamps in the final GeoJSON string.
It's more convenient for TimeDimension, because it uses only timestamps.

Julian days now refer to the current day instead of a fixed one.
Also handle Date objects.

Some conditions were applied on both single values and vectors using || and && operators.
It passed the tests, but caused errors on check-cran.

The check of R-cran consider a column name as a variable.
Therefore, ieeded to replace the subset function by another command.