Data quality checks - Githubissues

[x] Unit transformation. Create functions to transform between sap flow units. _Implemented in qc_sapw_conversion and other helper functions (qc_sapw_area_calculator, qc_get_sapw_md and qc_cm_cm_h family functions)_.
[x] Units visualization. Functions to visualize different sapflow units (sapwood, plant, leafarea) to detect incongruencies. Semiimplemented using dygraphs package
[x] Timestamp errors. Functions to identify tiemstamp errors. _Implemented in qc_timestamp_errors and qc_is_timestamp functions_
[x] Gaps. Functions to identify and characterize time gaps in the timestamp. Also functions to obtain information about gap coberture. _Implemented in qc_mind_the_gap and vis_plot_the_gap functions_
[x] ~Data aggregation. Functions to allow data aggregation based on time (daily aggregations for example)~ Not needed in this step
[x] Temporal series visualization. Functions/Interactive reports to allow the temp series visualization. Semiimplemented using dygraphs package
[x] Data load from csv. As csv files come in so many flavours, functions to ensure a correct data load are needed.
[x] Data columns classes. Numeric classes are to be expected in data so we need functions to detect the class and identify the causes of class mismatch.
[x] Calculate solar time and extraterrestrial solar radiation. _Implemented in qc_ext_radiation, but the results of the function regarding solar time should still be compared with external conversions to be sure it works properly_
[x] Function to check if there is concordance between sapflow TIMESTAMP and environmental TIMESTAMP. _Implemented in qc_timestamp_concordance function, both graphical and numeric outputs_
[x] Function to obtain t0 and tf for each tree, as well as for each environmental variable. _Implemented in qc_time_interval function_
[x] Visualization functions:
- [x] Heatmap calendar for fast identification of gaps. _Implemented in vis_gaps_calendar_
- [x] Histogram for gap lengths to see distribution. _Implemented in vis_plot_the_gap_

sapfluxnet / sapfluxnetQC1

Data quality checks #5