Missing pandas API functions

[nipsn]

Is your feature request related to a problem? Please describe.
It would be useful for users to be able to use their existing and tested pandas code, and at the moment there are functions missing implementation on the pandas API.

Describe the solution you'd like
An implementation of the pandas functions provided below that followed their original interface.

Describe alternatives you've considered

Additional resources
These are the functions without implementation that we could find. If more were to be found, they would be added to this list.

Function	Branch/PR	Done	Status
melt	pandas_melt		In progress (Marcos)
concat	-		-
get_dummies	pandas_api_get_dummies		In progress (Marcos Deprecated)
from_dummies	-		-
unique	-		-
isna	-		-
isnull	-		-
to_numeric	-		-
to_datetime	pandas_api_to_datetime		In progress (Marcos Deprecated)
to_timedelta	-		-
values	-		In progress (Tian)
empty	-		-
keys	-		-
isin	-		-
mask	-		-
Addition	-		-
Substraction	-		-
Element-wise multiplication	-		-
Dot multiplication	-		-
Less than	-		-
Greater than	-		-
Equal	-		-
map	-		-
rolling	-		-
clip	-		-
round	/pull/7		In progress (Oscar)
corr	-		-
kurt	/pull/6		In progress (Miguel)
count	/pull/2		1st PR
quantile	-		-
rank	-		-
sem	-		-
std	/pull/3		1st PR
skew	/pull/1		1st PR
nunique	/pull/9		Ready for review
add_prefix	/pull/8		1st PR
add_suffix	/pull/8		1st PR
at_time	-		-
between_time	-		-
id_max	-		-
id_min	-		-
sample	-		-

[neutropolis]

I've moved this table to a discussion page: #19