Update MMGIS API vector layer functions

[ac-61]

This removes the keepN parameter from the mmgisAPI.updateVectorLayer and creates 4 helper functions for removing existing features on vector layers. Splitting the trim functionality into separate functions solves the issue of whether or not we should include new input data when removing features.

The trimVectorLayerKeepAfterTime function removes features on a specified layer before a specified time:
window.mmgisAPI.trimVectorLayerKeepAfterTime(layerName, keepAfterTime, timePropPath)

The trimVectorLayerKeepBeforeTime function removes features on a specified layer after a specified time (i.e. this function does the opposite of trimVectorLayerKeepAfterTime:
window.mmgisAPI.trimVectorLayerKeepBeforeTime(layerName, keepAfterTime, timePropPath)

The keepLastN function retains the same functionality as the original keepN parameter in the mmgisAPI.updateVectorLayer function (i.e. removes features on a specified layer to keep the specified number of existing features):
window.mmgisAPI.keepLastN(layerName, keepLastN)

The keepLastN function is unaware of time so it will remove features based on the order the features were added to the specified vector layer.

The keepFirstN, the opposite of the keepLastN function, will remove features on a specified layer, with features removed starting from the tail end of the features list.

[tariqksoliman]

LGTM!

[tariqksoliman]

LGTM and thanks for the changes!

[ac-61]

@tariqksoliman @jtroberts I updated the pull request to add additional functions. window.mmgisAPI.trimVectorLayer is now window.mmgisAPI. trimVectorLayerKeepAfterTime and has a matching function to trim in the opposite direction window.mmgisAPI. trimVectorLayerKeepBeforeTime. window.mmgisAPI.keepLastN remains the same and now has an extra friend, window.mmgisAPI.keepFirstN.